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  1. 1. UNIT 2019 Know how to prepare surfaces for decoration 2 Surface preparation is very important for producing a high-quality finish. In this unit we will look at some typical surfaces you may find yourself working on during your career, along with appropriate preparation tasks for each surface. It is important that all surface contaminants such as dirt, oil, rust and loose or flaking existing coatings are removed. If these contaminants are not removed, it could affect whether the paint or paper will adhere (stick) to the surface. This unit also contains material that supports NVQ unit QCF 332 Prepare surfaces for Painting/Decorating. This unit also contains material that supports TAP Unit Prepare surfaces for Painting/Decorating. This unit covers the following learning outcomes: Preparing timbers and timber sheet products ready to receive finishing systems Preparing metal surfaces ready to receive finishing systems Preparing trowelled finishes and plasterboard ready to receive finishing systems Removing previously applied paint and paper ready to receive finishing systems Rectifying surface conditions Repairing and making good surfaces 97
  2. 2. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition K1. Preparing timbers and timber sheet products ready to receive finishing systems Timber is one of the most commonly used materials in construction. You will encounter timber in a wide range of Key terms situations, both internal and external. Botanical – the classification of trees based on scientific study Applications of timber and timber Deciduous – the name given sheet productsUnit 2019 Know how to prepare surfaces for decoration 2 to a type of tree that sheds it Types of timber leaves every year Timber is classified as either softwood or hardwood. This can Evergreen – the name given sometimes be confusing as not all hardwoods are physically to a type of tree that keeps its leaves all year around hard or softwoods soft. Hardwood and softwood refers to the botanical differences and not the strength of the timber. Hardwood trees are deciduous, broad leaved, with an encased seed. Softwood trees are usually evergreen with needles and seeds held in cones. Name Appearance Properties/Description Uses Redwood Moderately strong for its weight with Used for interior or exterior (commonly average durability. Quality of finish work and for carcassing and known as pine) depends on knots and amount of finish joinery resin. Capable of smooth, clean finish and can be glued, stained, varnished and painted. Whitewood Similar to redwood in strength and Similar uses to redwood (also known durability. Takes glue, nails and screws as European well and can produce a good finish. Spruce) Western red Not as strong as redwood but has Externally for good-quality timber cedar naturally occurring oils which prevent buildings, saunas, etc. insect attack. Doesn’t need treating as will stand up to severe weather and turns a silvery colour when exposed. Table 19.1 Commonly used softwoods 98
  3. 3. Unit 2019 Know how to prepare surfaces for decoration 2 Name Appearance Properties/Description Uses Oak Very strong with English oak the High-class joinery, panelling, doors, strongest. Good resistance to exposed roofing, etc. bending and shearing. Susceptible to fungi attack and ironwork should not be used as it will stain and disfigure. Beech Hard, close grained and durable Furniture, kitchen utensils, wood with a fine texture. Capable of a block floors, etc. good smooth surface. Takes glue, stains and polish well and can produce an excellent veneer. Unit 2019 Mahogany Strong for its weight and High-class joinery, furniture, boat moderately resistant to decay. building and plywood veneers Takes glues, finishes, nails and screws well. Know how to prepare surfaces for decoration 2Table 19.2 Commonly used hardwoods Name Appearance Properties/Description Uses Plywood Made from thin layers of timber Used in floors, walls and roofs glued together to form boards. Alternating grain across and along the sheet gives strength and stability. MDF Made from pulped wood mixed Skirting boards and mouldings with adhesives and pressed into sheets. Moisture resistant MDF is available. Hardboard Manufactured from sugar cane Flooring, furniture and units pulp mixed with adhesives and pressed into sheets 3–6 mm thick. Has a reasonable resistance to moisture.Table 19.3 Commonly used timber sheet products 99
  4. 4. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Uses of timber Timber has several different uses in the construction industry. You will be familiar with some of these uses, particularly structural uses, from Unit 2003, pages 81–85 and page 88. l Structural – floors, walls and roofs providing stability to the building. Timber is a core part of the construction or as decking over a framework. l First fixing – any work inside a structure before plastering takes place. Includes studwork, ground lats, stairs, windows and doors. l Second fixing – any work inside a structure after plasteringUnit 2019 Know how to prepare surfaces for decoration 2 has taken place. Includes all joinery work, such as doors, architrave, units and ironmongery. l Decorative – any structure made entirely from timber such as fire surrounds, mouldings, balustrades, banisters, dado rails, Safety tip flooring and decking/fencing. Knotting solution is highly flammable and so should not be exposed to naked flames. Defects in timbers and timber sheet You must also make sure you products, and treatments wear the appropriate PPE when handling this material. Timber can be affected by a wide range of defects. If you are not sure what preparation or decoration is required for a particular type of timber, you should always seek advice before starting the task. Knotting and resin exudation A knot is a place in the timber where a branch was joined to the tree. Resin may bleed from knots, staining the paint finish. Knotting solution seals knots and can be applied to areas stained with resin, tar and ink. Its main ingredient is shellac, produced by an insect and melted into thin flakes. Clean and dry the surface before applying with a brush. It should dry quite quickly, after which time the surface coating can be applied. Figure 19.1 Knotting bottle Stain sealing Shellac is also available coloured (known as pigmented shellac). Key term Aluminium provides a silver pigment while titanium provides a Resin – a natural liquid formed white pigment, and these are very effective stain sealers in wood when it is converted especially on: into useable timber. Very sticky and usually yellowish l stains made by fire, smoke and water l previously creosoted timber gold, under heat it emerges from knots (this is called resin l animal, smoke and fire odours (smells). exudation) 100
  5. 5. Unit 2019 Know how to prepare surfaces for decoration 2End grain, open grain and cracks Did you know?Wood grain is the growth rings found inside all tree trunks. When Shellac is not only used in staintimber is cut in the opposite direction of these rings this is called sealers. It can also be used as acutting ‘against the grain’ and it can cause timber to chip or tear. safe coating on foods, such as fruit and sweets, to give them aOther grain cuts include: glossy shine.l with the grain – cutting the timber is easier and cleanerl across the grain – cutting across the grain lines but the plane of the cut is still aligned with theml end grain – the timber is cut at right angles to the grain, for example when trimming the ends of planksEnd grain often needs to be repaired by the decorator by stopping Unit 2019and filling, which should be done after priming. A stopper is stiff Key termmaterial used to ‘make good’ gaps and holes on surfaces and it Open joints – gaps in timberdries with the minimum amount of shrinkage. Areas such as open structuresjoints and splits in timber need to be stopped and made flushprior to decoration.There are several different types of filling methods that you willneed to use. Know how to prepare surfaces for decoration 2l Proud filling – overfill and leave a raised amount of material filler (proud). After drying the filler will reduce or shrink back leaving it slightly proud. It will need abrading level.l Back filling – press the material deep into the area then leave it to dry. Repeat the process until the surface is level. Figure 19.2 Stoppersl Flush filling – use a filling knife or caulk board to apply the filler and make the surface flush prior to applying coatings.Filler used for good open-grained timberl Plastic woods – made from a mixture of resin and wood flour, used when applying clear wood finishes. Available in a ready mixed formula and two-pack formula. Very quick setting Key term but expensive. Flush – when one surface isl Two-pack stoppers – hard-wearing and can have fittings level and even with another screwed into them. Dry very quickly and have little shrinkage. surface Must be used on bare surfaces as they can cause coatings to become defective due to the ingredients in the stopper.l Putty – substance mixed with linseed oil which becomes sticky. Commonly used on wood for filling holes and to fix glass into window frames. Less expensive than other stoppers but becomes brittle with age.Moisture contentWet rot is a growth of brown fungus in damp timber. As thefungus grows, it destroys the wood. The only long-term treatment 101
  6. 6. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Find out of wet rot is the removal of moisture. Before working on an area Why should you not put iron affected by wet rot, you must treat it, as outlined here. screws into timber affected by l Rake out any defective timber. Allow the surface to dry and wet rot? flood the timber with a clear wood preservative. Allow it to dry and spot prime the affected areas with wood primer. l Fix wood screws (non-ferrous, that is non-iron) into the timber. l Apply a coat of two-pack polyester filler to the surface and allow it to dry. The screws will help the filler adhere to the surface. l Apply a second coat of filler and allow to dry. Abrade the filler so it is flush. An acrylic spot filler (a soft putty) may be neededUnit 2019 Know how to prepare surfaces for decoration 2 to fill any minor imperfections. Glue residue and nail heads Glue residue is excess glue left on the surface of timber. If dry, this can be removed with a scraper and then sealed with knotting solution. If wet, remove with scraper and clean with white spirit or turps and leave to dry. Nail heads may be left projecting above the surface. Place a nail punch squarely onto the nail head, covering the whole surface of the head, and hammer the nail further into the timber until it is below the surface. Apply a stopper or filler to the hole. Surface and physical properties of timbers and timber sheet products Tactility and aesthetics are the key characteristics of wood and timber. l Tactility – how workable the timber is to create different structures and fittings. The more workable it is, the more Figure 19.3 Tactility and potential uses it will have. Tactility also applies to other aesthetics: the workability and materials, such as plaster. beauty of timber l Aesthetics – the finishing look of timber when it has been stained and varnished. The more pleasant the aesthetic look, the more likely it is to be used decoratively. Porosity A porous surface is one that contains tiny holes through which liquids or gases can pass. In order to prevent porous surfaces, such as timber, from being penetrated by water or damaged by frost, a silicone water-repellent layer can be applied to waterproof the surface. When dry it is completely clear. 102
  7. 7. Unit 2019 Know how to prepare surfaces for decoration 2Physical propertiesTimber has a number of physical properties. Some of theseproperties also apply to other materials you will encounter –the qualities they give will be essentially the same for each typeof material. Insulation Ability to trap heat. Wood has low thermal conductivity so it is a natural insulator. Air pockets in the wood make it a barrier to heat and cold Figure 19.4 Silicone water Hardness Resistance to wear and tear. Also difficulty to saw or plane the timber repellent used on porous timber Strength Amount of stress timber can resist before bending. The ‘stiffer’ the wood, the stronger it will be Unit 2019 Flexibility Ability to expand and retract without damage Absorption Ability to absorb shocks and damage Adhesion Ability to ‘stick’ or fix to a different surfaceTable 19.4 Physical properties of timberCapillary action Know how to prepare surfaces for decoration 2This is a process where liquid is drawn up through a small gapbetween the surfaces of two materials. In a building this couldallow water to rise up from the surface and into the building.Timber can be affected by this if its moisture content is above30 per cent. Capillary action will also cause the timber to rot.This also applies to plaster and metal areas. These will not rot Rememberbut plaster will become weaker and metal will corrode Abrading a new softwood(see page 111). or hardwood may result in damage due to scratching or furring (the lifting of woodAppropriate abrasives fibres). For this reason, it is bestAbrading a surface means wearing away the top layer by rubbing, to simply dust off the surface prior to painting.or creating friction. This is a very important part of surfacepreparation. It provides a key for the coating or covering to beapplied and smoothes the surface to give a good-quality finish.It is important that the correct type of abrading material is used. Key term Key – roughness on a surfacel Abrading material that is too rough can leave scratches on provided to aid adhesion surfaces that show through to the finish.l Material that is too fine can have a long preparation time and be ineffective at removing or levelling rough surface imperfections.l Cheap and inadequate materials (such as glass paper) can greatly extend the preparation time as they get blunt and clog very quickly. 103
  8. 8. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Abrading materials and equipment Grades of abrasives It is important to select the correct grade of abrasive for each job to achieve the correct finish. The grade is printed on the back of abrasive paper and relates to the particles of aggregate to every square 25 mm. A grade that gives a coarse abrading effect will have large particles and therefore less of them. Figure 19.5 shows aggregate on a P20 grade dry abrading paper where only 20 particles of aggregate will Figure 19.5 A small number fit on to a 25 x 25 mm area. of large aggregates will give aUnit 2019 Know how to prepare surfaces for decoration 2 coarse abrading effect A grade that gives a fine abrading affect will have lots of small particles. Figure 19.6 shows aggregate on a P80 grade dry abrading paper where 80 particles of aggregate will fit on to a 25 x 25 mm area. Wet and dry abrasives Harcourt Education These can beJ6637 Paintingboth wet and dry conditions. A waterproof used in and Decorating adhesive fixes9pt Zurich BT the abrasive particles to the backing, which means AW031 that the paper does not lose the particles when it gets wet – in fact, if wet and dry paper is used dry it tends to clog up. The aggregates used in wet and dry abrasive paper are often silicon carbide, but aluminium oxide is becoming increasingly Figure 19.6 A large number of popular. Particles of aggregate are closely grouped together and small aggregates will give a fine abrading effect referred to as being ‘closed coated’. Water (sometimes mineral oil) can be used as a lubricant, preventing paper from becoming clogged. Wet and dry abrasive is available in grades from P80 (coarse) through to P1200 (very fine). Remember Advantages Disadvantages Always choose the correct type Extremely good for Harcourt Education high-quality work More expensive than some dry abrasives and grade of abrading material J6637 Painting and for the surface and the job. Wide range of grades available Decorating Unsuitable for bare timber 9pt Zurich BT AW032 Cleans the surface as it abrades Clogs up easily if used dry Low dust levels Surface must be dry before decoration Safety tip Abrading will create dust Table 19.5 Properties of wet and dry abrasives particles so ensure adequate ventilation of the work area and Dry abrasives wear appropriate respiratory These use a non-waterproof adhesive to fix the abrasive particles PPE. Some tools are equipped to the backing paper. The best aggregate to use in this type of with a dust collection bag. paper is aluminium oxide grit. Glass and garnet are common, but less effective. 104
  9. 9. Unit 2019 Know how to prepare surfaces for decoration 2Aluminium oxide abrasive (production paper) is usually available‘open coated’, where the particles of aggregate are spaced apart New abrasiveon the backing paper. This reduces the risk of clogging as thegaps allow waste to escape. Dry powder lubricants can be used onsome types of dry abrasives, breaking away when heat is generated Aluminium oxide particlesby the abrading process, preventing clogging of the abrasive. wear down and break awayDry abrasives are available in grades ranging from P20 (coarse)through to P320 (very fine). Remaining particles are smaller and Advantages Disadvantages sharper When worn, particle edges shear off Aluminium oxide can be expensive Figure 19.7 How aluminium revealing smaller but sharper edges compared to other abrasives oxide breaks down Unit 2019 (Figure 19.7) Available in sheet, roll, disc and belt form High dust levels produced Available in self-adhesive rolls – abrasive can be torn off and fixed to purpose-made rubbing blocksTable 19.6 Properties of dry abrasives Figure 19.8 Belt sander Know how to prepare surfaces for decoration 2Mechanical sandingElectric tools can greatly reduce time spent preparing surfacesand increase the surface area covered. Electrical sanders work bymoving an abrasive pad or belt at a fast speed.Belt, drum and orbital sandersThe heavy duty sanders most commonly used by a decorator arebelt, drum and orbital sanders.Belt and orbital sanders are hand-held power tools best used forsanding large, flat items of joinery. A drum sander is self-propelledand used for stripping floors. A rough grade of abrading materialshould first be used to remove surface coating. The surface canthen be brought up to a smooth finish by progressively using finerand finer abrading material. Advantages Disadvantages Figure 19.9 Drum sander Effective at abrading large areas More expensive than abrasive papers Faster rate of abrasion than by hand Only suitable for large, flat areas Can create large amounts of dustTable 19.7 Properties of belt and orbital sanders Figure 19.10 Orbital sander 105
  10. 10. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Did you know? Disc or rotary sanders Small electric sanders are also Rotary sanding involves the use of rotating discs of abrasive available with triangular heads material and can be used to prepare small or contoured surfaces. for use when sanding corners. Different types of abrasive disc are available: Abrasive discs can be fitted to electric drills and angle grinders. l flat discs that require a backing pad l flap discs made up from flaps of abrasive, which are more expensive but also more effective l grinding discs that can be used for removing very heavy, small areas of rust. Advantages DisadvantagesUnit 2019 Know how to prepare surfaces for decoration 2 Do not burnish the surface Only suited to small areas Effective at removing isolated patches Not suited to complex surfaces (discs of rust cannot reach into awkward corners) Figure 19.11 Disc (or rotary) Relatively low initial cost of equipment sander – in this instance, an electric drill fitted with an Table 19.8 Properties of disc or rotary sanders abrasive disc attachment If the sander is equipped with a dust collection bag, make sure it is working and empty it before using the tool. After sanding, the wood dust collected should be disposed of appropriately. Sanding dust should not be left in bags indoors as there is a danger of it catching fire through its own heat. Appropriate solvent-based and water-based primer Priming is the first coat of paint applied to a surface. Primers protect the substrate and give an even and consistent finish to the final coat. Universal primers are designed to be used on a range of surfaces. If the surface preparation or the application and choice of the primer is incorrect in any way, the durability of the paint system will be reduced. l Solvent-based primers – form a waterproof layer to prevent wet rot in timber surfaces. They do not raise the grain of the wood when applied, but have a longer application time than water-based primers as the solvent content prevents them from drying as quickly as water-based primers. l Water-based primers – provide a moisture screen to the surface. Water-based primers do not soak into timber, meaning they adhere less well than oil-based paints. They are low in VOCs and odour, quick to dry and more durable on hardwoods. 106
  11. 11. Unit 2019 Know how to prepare surfaces for decoration 2 Primer Description Acrylic primer Water based. Reapplied as an undercoat to speed up coating process. Mostly used for internal timbers but can be used externally. Aluminium wood primer Solvent based. Has aluminium non-leafing particles to make it more suitable for priming resinous timbers such as Columbian and Oregon pines. Can be used for both internal and external timbers. Wood primer Solvent based. Can be used on softwood and hardwood internal and external timber surfaces. Preservative primer Solvent based. Used for external timbers only. Similar to wood stains and varnishes as it protects the timber Unit 2019Table 19.9 Types of primerCorrect preparation process forrectifying defects in timberThere are different preparation techniques for each type of timbersurface. Follow the techniques described earlier in this unit, aswell as remembering the different qualities of types of timber, Know how to prepare surfaces for decoration 2before you begin work.Bare untreated timberFor basic painting tasks:l seal any knots in timber using knotting solutionl prime the surface using oil-based wood primer (for external surfaces) or acrylic primer undercoat (for internal surfaces)l fill using polyfiller and decorator’s caulk, rub down and dust offl apply one coat of undercoat, rub down and dust offl apply another coat of undercoat if necessary then apply one coat of glossAlternatively, for staining or varnishing tasks:l fill holes in timber with putty or coloured stopper and apply a base coatl rub down and dust offl apply one coat of wood stain or varnish and lightly rub down and dust offl apply second coat of wood stain or varnish. 107
  12. 12. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Previously painted timber l Rub down using sandpaper. If necessary, fill using polyfiller or caulk. l Apply one coat of undercoat, rub down undercoat and dust off. l Apply one coat of gloss. Rough cut timber Rough sawn timber should be dry brushed thoroughly to remove soil, vegetation and dust. l Apply one coat of timber preservative or wood stain. l Apply a second coat of timber preservative or wood stain.Unit 2019 Know how to prepare surfaces for decoration 2 K2. Preparing metal surfaces ready to receive finishing systems Metal has a wide range of uses throughout buildings and structures. It can be used for frames, girders and trusses. Some Key term metals, such as aluminium, are also used for decorative reasons. Element – a substance There are several different types of metal that you may encounter that cannot be broken down into any other substance. as you work. Metals are either pure or alloys. For example water is not an l Pure – the metal is made from only one element. Common element as it can be broken examples of this include gold, silver, lead, copper, aluminium, down into hydrogen and zinc, iron and tin. oxygen. These cannot be broken down so are elements l Alloy – a mixture of two or more metal elements, used when a strong, light metal with properties that do not exist in a pure metal is needed. Aluminium alloy is a common example of this. Surface and physical properties of metal types Metal shares some qualities with timber, including porosity (page 102). The other key qualities of different types of metal are shown on page 109. Metals will not exhibit all these qualities. 108
  13. 13. Unit 2019 Know how to prepare surfaces for decoration 2 Quality Description Malleability Can be hammered and pressed into different shapes Conductivity Has the ability to conduct heat and electricity Elasticity Can regain its shape after being misshapen during use Hardness Resistant to scratches and cuts during construction Brittleness Extremely hard but can be broken very easily Ductility Can be stretched without breaking and turned into a fine wire. Find out Metals with this quality are used for metal cables. Use the Internet as well Toughness Can absorb shock and energy without breaking (the opposite as materials lists and Unit 2019 of brittleness) manufacturers’ information Tensile strength Tested under extreme conditions (pulling, squashing, twisting and to find out more about shearing) and can withstand these forces the circumstances where these qualities of metals are Heaviness Denseness of the metal in relation to its size particularly desirable. Strength Tested to withstand heavy loads without breakingTable 19.10 Key qualities of different types of metals Know how to prepare surfaces for decoration 2Types of metal and their applicationsNon-ferrous metalsNon-ferrous metals do not contain iron and are not magnetic.They are usually more resistant to corrosion as they havenon-friable oxide layers, created by the atmosphere. These shouldbe dry and free from grease prior to painting. Previously paintednon-ferrous metals need to be abraded and any corrosion depositsfound should be scraped back to a firm edge where any flakingpaint is evident.Ferrous metalsFerrous metals contain iron and may have small amounts of othermetals and elements added to them to give them the propertiesthey need. Most ferrous metals are magnetic. These surfacesare prone to rusting and will need to be cleared of all rust priorto painting. Depending upon the extent of the rust, it can beremoved with the use of a wire brush, mechanical wire brush,abrasive papers and/or scrapers. 109
  14. 14. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Name Appearance Properties/Description Uses Copper Extremely ductile, malleable, good Available in tubes, sheet, (non-ferrous) conductor. Tarnishes and oxidises quickly. wire, rod and flat bar. Used Easily damaged and must be stored for water pipes, electrical carefully. wiring and roofing. Aluminium Extremely malleable, ductile, lightweight Some types of window (non-ferrous) and conductive. Non-toxic and often frames. Excellent for alloyed. Highly resistant to corrosion. stamping and forming. Can be dyed (anodising).Unit 2019 Know how to prepare surfaces for decoration 2 Lead Very soft, malleable, heavy and highly Roofing (non-ferrous) resistant to corrosion. Tarnishes to a dull grey when exposed to air. Very poor conductivity. Poisonous, so care must be taken. Galvanised Highly resistant to corrosion, as alloyed Girders, frames, roofing, steel with zinc to protect the iron. Can support beams, piping, etc. (ferrous) withstand saltwater, moisture, rain, snow, Available in tubes, sheets, etc. Lightweight, fire-resistant, basically ropes and flat bar. maintenance free and extremely durable and resistant to scratches and abrasion. Cast iron Corroding metal, non-toxic, made by Bridges, buildings, stairs, (ferrous) melting pig iron and small amounts of handrails, cast iron columns, scrap steel. Strong, hard, self-lubricating items such as machinery parts and brittle but also cheap, well wearing and sustains heat. Wrought iron Iron alloy with very low carbon content Roof trusses, ornamental (ferrous) made by melting porous iron with slag ironwork, pipe work, and other impurities. Gives it properties handrails. Available in bar not found in any metal. form, sheets, rods and hoops. Tough, malleable and ductile. Can crack if bent or heated up and brittle when cold. Has a rough texture so it can hold platings and coatings. Mild sheet/ Iron alloy, corrodes and has high carbon Girders, tubes, screws, nuts steel content so vulnerable to rust. Malleable, and bolts and garage doors (ferrous) ductile and tough with high tensile (use composites for wood strength and bends easily. apppearance). Table 19.11 Non-ferrous and ferrous metals 110
  15. 15. Unit 2019 Know how to prepare surfaces for decoration 2Corrosion Key terms Anodising – anDuring your inspection of the work surface, you may notice areas electrochemical process thatwhere the surface has corroded, usually due to rust. This will have converts metal surfaces into ato be cleaned and removed before work can be carried out. decorative, durable, corrosive- resistant, anodic oxide finish.Main corrosion factors Aluminium is the metalCorrosion is the destructive attack on a metal from its often used, but titanium andenvironment. The main corrosion factors are all found in the magnesium can also be used. These metals are immersed inatmosphere. They are: an acid electrolyte bath with anl oxygen electric current running throughl hydrogen the medium Unit 2019l moisture Pitting – formation of small pits in a metal surface as al pollutants. result of corrosion. CreviceThe most common form of corrosion is oxidation. The atoms in corrosion occurs wheremetal combine with the atoms in oxygen to form oxides. Iron rust nuts, bolts and gaskets haveis the most recognisable form of corrosion and is caused by iron been usedoxide appearing on iron or steel components. Metals with highiron content will corrode more than metals with low iron content. Know how to prepare surfaces for decoration 2Metals which show signs of corrosion, such as pitting, must berepaired and the metal protected with a coating.Corrosion can protect some metals, such as copper.Copper is used as both a pure metal and an alloying material.Because it is very resistant to corrosion, it does not need tobe treated with protective coatings as most other metals do. Itdevelops a protective oxide coating. This thickens the copper andturns it green. Did you know? On iron and steel a thin, flakyGalvanic corrosion and cathodic protection black iron oxide called millscaleElectrolysis is an electrochemical process, where a metal comes can form (see page 113). Thisinto contact with an electrolyte (a conductor, usually water) is an example of cathodic protection to the steel.and parts of the atoms of the metal (electrons) flow from themetal into the electrolyte, causing it to corrode. This is calledthe galvanic action. If two metals are in the same environment,the metal that has less resistance to electrolysis (the anode) Find outwill corrode before the other metal (the cathode). Some metal Use the Internet and othercoatings have metal in them lower than the actual substrate resources to try and find somebeing painted. This provides protection to the substrate, as the examples where electrolytic corrosion is used.anode in the coating causes it to corrode first. This is known ascathodic protection. 111
  16. 16. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition To use an example from Table 19.12, if iron sheet has a zinc coating, this will protect the iron as the zinc will rust first. The zinc is less resistant than the iron and therefore is the anode. Copper Tin Lead Nickel Iron Zinc AluminiumUnit 2019 Know how to prepare surfaces for decoration 2 Magnesium Table 19.12 Metals in order of their resistance to corrosion Appropriate primers and their function Primer Description Mordant solution Also known as etch primer or T-wash. Very toxic and used for Key term non-ferrous metals. Must have an overcoat after application to avoid deterioration. Available in two-pack coatings, which Blast cleaning – an have better stability and adhesion but a limited shelf life once alternative way to remove mixed. Touch dry within 1–4 hours and can be recoated in 10 corrosion and coatings from to 14 hours. steel or metal work. It involves Metal primer Specially formulated to prevent rust and provide adhesion, grit particles being shot through (acrylic-based) low VOCs, non-toxic, odourless. Expensive to purchase and a hose under high air pressure, can only be used on properly prepared surfaces. Can be which removes everything it recoated in 4 to 6 hours. hits on surfaces. This method is used in heavy industry and in Zinc phosphate Solvent-based primer with a rust-inhibitive pigment, touch-dry within to 3 hours. Suitable for non-ferrous metals, fabrication shops. iron, steel and blast-cleaned surfaces. Table 19.13 Primers and their functions Preparation processes for metal Solvent wiping Solvents (for example, white spirit and turps) remove grease and oil from metal prior to decoration. Solvents are used to avoid rust on the surface. Solvents are very toxic so make sure you are in a well-ventilated area and that you are wearing the correct PPE. 112
  17. 17. Unit 2019 Know how to prepare surfaces for decoration 2Removing rust by handCleaning off rust by hand is normally done when repainting rustysteelwork, as it is usually the cheapest method. The problem withhand cleaning is that the use of scrapers, chipping hammers,wire brushes and abrasives will not remove all traces of rust. Inaddition, the overuse of a wire brush can serve only to polish therust on the surface, affecting the ability of the primer to adhere tothe surface. Figure 19.12 A wire brush can be used to remove loose rustFollow this procedure when cleaning by hand.l Remove any traces of oil or grease to avoid spreading it around the surface. Unit 2019l Scrape off all loose rust, millscale and previous coatings. Key termsl Use a chipping hammer around rusted nails, bolts and rivets. Millscale – a thin flakey blackl Use a wire brush to remove loose rust, but avoid burnishing. iron oxide formed on ironl Finish off by abrading with a rough aluminium oxide abrasive – and steel. It is also a cathodic P40–P60 (see page 104–106). protection to the steelRemoving rust with power tools Burnishing – polishingPower tool cleaning is generally quicker and more effective than Know how to prepare surfaces for decoration 2hand cleaning and will extend the life of the paint system. Looserust, millscale and the existing surface coating can be removedusing power wire brushes, grinders and needle guns, althoughsome millscale will not be removed even with power tools.Again, care should be taken not to over-polish the surface or theadhesion of the primer will be negatively affected.Follow this procedure when cleaning with a power tool.l Remove any traces of oil or grease from the surface. Figure 19.13 Power toolsl Scrape off all loose rust, millscale and previous coatings. such as needle guns and anglel Use a needle gun to remove rust around corroded nuts, bolts grinders can be used to remove rust from surfaces and rivets, etc.l Select the most effective method of removing rust to suit the nature and condition of the surface (for example, rotary wire brush, disc sander or angle grinder).Steelworkl Dry abrade using emery paper or a scraper and wire brush, and dust off.l Apply good general purpose metal primer or zinc phosphate to areas where rust has been removed (apply a full coat for previously painted steelwork).l Apply undercoat.l Lightly abrade, dust off and apply a gloss coating. 113
  18. 18. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Key term Other metal surfaces Bitumen – a heavy, semi-solid, Ferrous metals (iron and steel) brown-black substance created as a result of the oil refining l Remove all corrosion and millscale via mechanical means. process (also known as asphalt l Degrease with white spirit if necessary. or tar) l Allow the surface to dry thoroughly and apply primer with a brush. l Bitumen-coated surfaces will require sealing with shellac knotting solution or aluminium primer. l Four coats of paint will be required to achieve adequate film thickness (as recommended by the British Iron and Steel Association).Unit 2019 Know how to prepare surfaces for decoration 2 Non-ferrous metals (aluminium, copper, zinc, brass etc.) l Degrease surface with white spirit. l Galvanised and zinc-sprayed surfaces should be treated with mordant solution. l Etch the surface with wet and dry abrasive paper and white spirit to provide a key. l Apply one coat of metal primer or universal primer. Working life Jamila has given a client an estimate for repairs to discoloured, flaky and rusty metal railings. She states that the railings can either be removed and abrasive blasted, then treated and a full paint system applied or they can be prepared by hand and spot primed before a paint system is applied to them. There is a cost difference between the two because of the timescales involved. Which system should be chosen? What has caused the deterioration of the metal railings? How would the metal be treated after being blast cleaned? What suitable paint system could be used? Which estimate should the client go ahead with? K3. Preparing trowelled finishes and plasterboard ready to receive finishing systems Did you know? Plasterboard is a durable and high-quality lining for walls and Plasterboard can also offer extra ceilings, lift shafts and stairwells, corridors and auditoriums. moisture protection as it has Trowelled finishes are surfaces constructed from bricks and moisture controlling and blocks. Most walls use bricks and blocks as a central part of water-resistant properties. their construction. 114
  19. 19. Unit 2019 Know how to prepare surfaces for decoration 2Types of surface and applications Key term Gypsum – a white rockPlasterboard produced as the by-product ofMost plasterboard is made from gypsum, processed into a board industrial processesand given a paper covering. Standard plasterboard is suitable formost applications and is compatible with direct decoration orplaster finishes. It has a grey facing and an ivory coloured back.Plasterboard is used in a range of residential and commercialbuildings.Dry lining is where dividing walls in a building are made fromplasterboards (square and feather-edged) which are attached totimber structures (stud walls) and taped up. The joints of the two Unit 2019boards are sealed with joint tape with the joint then plastered overfor a smooth finish.BlockworkConcrete blocks are heavy but produce strong finished work.They are used where a lot of weight will be put on top of, or Figure 19.14 Plasterboardagainst, the wall. They are also used to form footings belowground on walls that support steel. Lightweight blocks are lighter Know how to prepare surfaces for decoration 2versions of concrete blocks, produced in response to health andsafety restrictions on lifting and handling units heavier than 20 kg. Block Appearance Uses Solid block Concrete block used for making walls above ground for commercial, industrial and leisure buildings. Also used for beam and pot floors. Hollow block Concrete block used where reinforcement is needed. Same finish as solid blocks but with hollow sections running through them. Filled with vertical reinforcement rods and concrete, making them very strong. Aircrete block Lightweight block made of a microcellular composition (aircrete). Lightweight but very durable. Used for foundations, beam and block floors and internal and external cavity walls.Table 19.14 Types of blocks and their uses 115
  20. 20. Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition Brickwork Bricks are smaller than blocks and so more are required per er square metre. A brick is 215 mm long, or half the length of 65 mm Hea der Stretch a block, and 65 mm high. The length of a brick is called the mm 102 .5 m 215 stretcher and the end of a brick is called the header. m Like blocks, bricks are held in place by mortar, a mixture of sand, Figure 19.15 Sizes of brick, cement and water used for bedding and jointing. Bricks, like Client: Harcourt Job No: J6598 header and stretcher blocks, need to be laid level and straight with equally sized joints Fig No: AW078 to achieve a sound wall with a good appearance. Physical properties of plaster andUnit 2019 Know how to prepare surfaces for decoration 2 trowel surfaces Plaster, brickwork and blockwork share a number of physical properties with metal and timber. Plaster has qualities of capillarity, tacility, adhesion and porosity. Brickwork has qualities of capillarity and porosity. Property Description Key terms Acidity Found in some surfaces and can cause defects if the surface has Alkaline – having a pH not been prepared and primed correctly greater than 7 (an acid has a pH of less than 7) Inertness Refers to plaster being able to bond to surfaces without reacting or causing defects Saponification – a chemical Soluble salt content Salt found in brickwork. It must be removed correctly or will reaction that makes soap and keep returning and lead to efflorescence (see page 117) so foams up as a result Permeable – allowing things Table 19.15 Some physical properties of plaster, bricks and blocks to pass through Alkalinity The chemical nature of surfaces such as concrete, cement Acrylic coating rendering, asbestos sheeting and some plasters is alkaline. This can cause problems if a solvent-based paint is applied Alkali surface as the alkalinity in the surface can attack the paint, causing Alkalinity permeating saponification. coating To prevent this, you should apply an alkali-resistant primer. This forms a barrier between the surface and the paint. The permeable nature of acrylic surface coatings means they need an alkali-resistant primer to prevent alkalinity coming through if the Figure 19.16 Alkalinity surface becomes damp. permeating through an acrylic coating Harcourt Education J6637 Painting and Decorating 9pt Zurich BT AW034 116
  21. 21. Unit 2019 Know how to prepare surfaces for decoration 2Defects associated with plaster andtrowel workLike timber, plaster and trowel work can suffer from cracks, nailheads and open joints. Moisture must also be removed fromplaster and trowel surfaces to avoid mould growth and many ofthe defects covered below.Settlement cracks, shrinkage and dry out Figure 19.17 Settlement cracksSettlement cracks appear in plaster and cement work in floors andceilings. They are caused by shifts in the elevation of a structurecaused by shifts in the soil. The soil may not have been compacted Unit 2019properly before construction and, as it decays, collapses – leavinga void beneath the building.Shrinkage is caused by material drying out after completion andcracking as it shrinks. In cement, if too much water is added tothe mix then it can shrink once this water has evaporated.Plaster that has been properly adhered will avoid shrinkage.Efflorescence Know how to prepare surfaces for decoration 2Efflorescence can be seen as the white patches on cement-basedsurfaces and it can occur on brickwork and plaster. Becausecement is porous, moisture can penetrate it, dissolving someof the lime and creating calcium hydroxide. This then rises tothe surface when the cement dries out, leaving white patches ofcalcium carbonate.Efflorescence will have to be removed before decoration. Scrubthe surface with a stiff fibre brush or a wire brush. Never removeefflorescence by washing the surface as the calcium carbonate willdissolve in the water and sink back into the cement.Defective rendering and raking outRendering is a coating of plaster applied to stonework. Cracks can Figure 19.18 Efflorescenceform in this.To repair small cracks:l scrape away any loose coatings and particles of masonryl apply filling agent – exterior grade filler (polyfiller type) could be used but this would probably re-crack after a short period of time, whereas exterior acrylic caulking will provide more permanent flexible repair. 117