In this chapter you will be learn about how painting is done.

1. Application and
Applications

2. Introduction
Coatings are materials applied to all manner of surfaces to
• Decorate them
• Protect them
from the environment and other sources of harm.
Sometimes they perform special functions such as
• Providing an anti-fouling surface.
• All that from a film rarely more than a few millimetres thick,
and usually only a few tens of microns.
• What is more, the coating must deliver its benefits with the
minimum impact on the health of those exposed to it,or the
wider environment.
• The application process is usually carried out by the owner of the
objects to be painted, or his agent, rather than the coatings
manufacturer.

3. SUBSTRATES AND THEIR PREPARATION
• Nature of the substrate
• Preparation of the substrate
Familiar materials of construction
Steel, wood, aluminum, plastics and
concrete
The most widely coated substrate is
paint itself since most successful
schemes are multi-layer

4. PROCESS OF PAINTING
Surface Preparation-
• Surface preparation is important because it directly affects the surface
finish of the paint.
• A paint scrapper, a putty knife or a sand paper is used to remove loose
particles or flakes and the surface is made enough and free from dust,
dirt, grease etc.
• All loose material is removed and filled with plaster or putty mixture.

5. • It is essential, especially when the walls to be painted are stained,
porous, have water damage or when we are painting a light colour over
a dark color or vice versa.
• Primers help provide a smooth surface, making the surface less
absorbent, thus increasing the spreading capacity of the paint.
• Using primer on the kitchen, bathroom and exterior walls is especially
important to aid in the prevention of mildew.
• Primer helps give uniform colour and texture to the finished paint, and
provide better adhesion to the top coats.
• The type of primer used is also dependent on the kind of surface to be
painted, i.e. different primers exist for
1. interior walls,
2. exterior walls,
3. wood, ‘
4. metals etc.
Primer coating

6. Putty Coat :
• Putties are generally used in the case of fresh painting or walls
that require total overhauling just like fresh walls, they may
not be used for repainting or touch up exercises.
• Wall putty should be applied to walls after first primer coat
has dried.
• It will fill in minor cracks and surface imperfections, and
provide a protective sub-layer for the expensive paint.
• As per the requirement 1-2 coats of putty can be applied, but it
is required to allow the previous coat to dry before applying
subsequent coat.

7. Second Primer Coat :
• Generally repainting services require a single
coat of primer, but for fresh surfaces or the
surfaces that need considerable leveling and
removal of undulations, two primer coats are
required.
• Care must be taken that the second primer coat
be applied only after the previous putty coat has
dried completely.
Finish Coat :
• Generally two coats of finish coat are required to
get the desired finish and color.
• It is important that each coat must be completely
dry before the application of next one.

8. Substrate preparation
Cleaning to remove
• Dust,
• Rust,
• Grease,
• Moisture and
• Loosely adhering paint residues.
By
• Abrasive paper,
• Chemical paint strippers and
• Detergents

9. Large steel structures
• Prepared by grit or shot blasting, when abrasive
particles are fired at them by compressed air or
pressurised water.
• The surface is not only cleaned, but also to some
extent reshaped to provide the asperities that enable
a strong mechanical bond to be formed with the
coating.
• It is important to apply a coating as soon as possible
after the blasting to prevent re-oxidation of the
surface.
• In shipyards, the blocks from which the ships are to
be assembled are blasted under cover and coated with
a silicate ‘shop primer’ before being taken outside

10. Smaller metal objects
• Subjected to a ‘conversion’ process.
• Car bodies, for example, are usually
‘phosphated’.
• Immersion in phosphoric acid produces a layer
of iron
• phosphate, whilst acidic zinc phosphates
produce a dense, uniform layer of mixed zinc
and iron phosphate crystals adhering strongly
to the surface.
• They have some protective effect in their own
right and provide an ideal substrate for paint
adhesion.

11. Plastics substrates
• Plastics substrates are very diverse and present significant difficulties to
the coater.
Polyethylene and polypropylene
• are difficult to wet.
• It is necessary to oxidise their surfaces chemically, or by flame treatment, if
a coating is to have any chance of adhering.
• Others are often contaminated by mould release agents that have to be
removed by detergent or solvent, whilst avoiding the danger of solvent
crazing that affects some plastics.
• Yet others may be significantly soluble in the paint solvents.
Elastomeric polyurethanes
• These are very flexible and require coatings of excellent flexibility.
• The impact resistance of components such as car bumpers can be
compromised by the application of coatings.
• Not only is solvent crazing a possibility, but also a brittle surface layer may
support the growth of cracks that develop sufficient velocity to continue into
the substrate.
• Stoving coatings, of course, have to be designed with due regard to the
thermal susceptibilities of the plastic substrate.

12. Coatings
• Coatings themselves often present difficult surfaces for coating.
Epoxies
• often have a narrow ‘overcoating window’ during which the next
layer can be expected to adhere well.
• If aged beyond the determined interval, it may be necessary to
prepare the surface by sanding.
Crosslinked coatings
• that have not aged for long enough may swell under the
influence of the solvents in the next coat and produce a severely
wrinkled surface.

13. • Brushes
• Rollers
• Dipping
• Electrodeposition
• Autophoresis
• Pneumatic Spray
• Airless Spray
• Electrostatic Spraying
THE APPLICATION OF COATINGS

14. THE APPLICATION OF COATINGS

15. Brushes

16. Brushing
- This is the simplest method and also the
slowest and most expensive.
- Promotes better wetting of the surface and
can be used in restricted spaces, be useful
for small areas, with less wastage and
contamination of surroundings.

17. Brushes used for Finishing
• Two main brushes used are
1. Bristle brushes and
2. Foam polybrushes
• Natural bristle brushes- are made with hog hair, commonly
used with oil base paints.
• Synthetic bristle brushes- are made of nylon or polyester,
commonly used with water soluble finishes, and most oil base
finishes.
• Foam polybrushes- made of plastic foam tapered on both sides
to a sharp point, can be used on most finishes

18. Rollers

19. Roller
• Brushes are very versatile
and effective but rather slow,
and large, flat surfaces can be
coated much more rapidly by
hand-roller.
• Large rollers are also used for
applying coatings in a
continuous process, for
example, to steel coil, and in
various printing processes.
• Reverse rollers operate
against the travel of the coil,
so their action has more in
common with a ‘wiping’
process.

20. Pneumatic
Sprays

21. Air Spray
• The earliest spray technique used a jet of compressed air at 2–3 atm to
‘atomise’ the paint.
• Small particles settled on the workpiece and coalesced to form very
smooth, uniform films (when applied by an expert).
• Pneumatic sprays are used, for
example, in car-refinish markets
and by the rather more
adventurous DIY painter, being
simple to use and robust.
• Their rate of coverage is rather
limited however (0.25–0.5 L/min)
and
• they are most effective with paints
of low viscosity (0.05–0.1 Pa s).

22. • Sometimes called
conventional spray
painting, this method
uses compressed air to
force the paint through
a nozzle into a fine
spray.
• This method usually
requires more solvent
than other methods,
making a significant
fire hazard.
Compressed air application

23. • A spray area is any
area where flammable
and combustible
materials are sprayed.
• Usually this refers to a
designated area
outside of a spray
booth or room.
• Spray painting indoors
but outside of a booth
or room is not
considered acceptable.
• A booth or room is
required for
production spray
finishing.
Spray Areas

24. Design and construction:
• Ignition sources: Designated spray areas need to be
located where there is no risk of vapors igniting.
• Examples of ignition sources
include:
− Non-explosion proof electrical equipment (lighting
and outlets)
− Welding or open flames
− Grinding (sparks)
• Only special explosion proof wiring and fixtures
approved for hazardous locations are considered safe
where flammable and combustible gases are present.
− This type of electrical equipment will not ignite
vapor-air mixtures.
Spray Areas

25. • Hot surfaces
− It is important that hot surfaces are
not located in spraying areas where
deposits of combustible residues
accumulate.
− An example of a hot surface is a
steam pipe.
• Minimum distance
− Ignition sources must be controlled
within 20 feet of a spray location.
• Sprinklers
− Sprinklers or other automatic
extinguishing equipment are
necessary for spray operations, as
well as ducting systems.
Spray Areas

26. Spray Booths
Spray booths:
• A spray booth is an enclosure with an open face which is
mechanically ventilated.
• A booth may use baffles, dry filters, or a water washing (waterfall)
system to remove paint overspray before it enters the exhaust duct.

27. Design and construction:
• There are numerous design and construction requirements for spray
booths.
• Requirements have been established for the purpose of fire and staff
protection.
• Construction:
− Booths must be constructed of fireproof material:
• 18 gauge steel
• Concrete
• Masonry
− The exhaust duct must also be constructed
of non-combustible materials.
− Combustible floor surfaces are allowed (but
not advised) so long as they are covered
with non-combustible material such as steel
plates.
Spray Booths

28. Spray Booths
Proper ventilation for spray booths:
• Spray booths with dry filters need to have a flow rate across the
open face of the booth of 100 feet per minute (fpm).
− For electrostatic spray operations a minimum of 60 fpm is
required.
• Air intake openings to spray rooms or rooms containing booths
need to be adequate to allow sufficient exhaust ventilation.

29. Spray rooms:
• A spray room is a fully enclosed room with mechanical exhaust.
• It is usually used for the spray painting of large objects such as
automobiles.
Spray Rooms

30. Design and construction:
• In general, the design and construction requirements for spray
rooms is the same as for spray booths:
− Rooms must be constructed of substantial non-combustible
material having a fire resistive classification.
Spray Rooms
− Other booth requirements
mentioned need to be followed,
e.g., electrical, grounding,
lighting, sprinklers, etc.
Proper ventilation for spray rooms:
• Ventilation in spray rooms must
be adequate to allow a minimum
of 30 air changes per hour.

31. Dipping

32. Dipping
• The development of dipping processes
followed the need to paint complex
objects on a large scale.
• It initially seemed the ideal way to coat
hidden areas of car-bodies, and other
recessed areas, if holes were provided in
enclosed sections to admit and drain the
paint.
• However, it proved to be much less
effective than expected, as the refluxing
of solvents in such areas tended to wash
the newly applied paint away.
• Variants on the dipping process involve pumping liquid paint over suitable
objects (flow or flood coating) or
• passing the workpieces through a sheet of falling paint (curtain coating).
• All are low shear processes.
Dip coat application – Parts are immersed into a tank of coating followed
by drainage of excess coating back into the tank.

33. Fluidized Bed Coating
• Polymer powder in air generated fluidized bed
• Normal or electrostatic FB coater
• PVC, polyethylene copolymers
• Protective, decorative coatings
• Safety glass bottles
Heat part, then dip into FB
coater
100 °C for PE copolymer

34. Fluidized Bed Coatings
A part recently taken out of a fluid bed coater with about 10 mils of
functionalized polyethylene applied.

35. Dip Coating
PVC

36. Airless
Spray

37. Airless Spray
The efficient painting of large
substrates such as
1. Ships,
2. Bridges and, to some extent,
3. Buildings
requires a higher rate of delivery of
paint than pneumatic spray permits.
• Airless spraying answers the need,
being able to deliver paint of 0.5 Pa-s
at a rate of 2 L/min.
• Paint is compressed at up to 350 atm (using compressed air to drive a
pressure ‘intensifier’) and forced through narrow nozzles, where
expansion provides the atomisation and propulsion towards the substrate.

38. Airless application:
• Paint is forced through a small opening under very high
pressure causing a spray.
• Smaller amounts of solvent are used.
• The high pressure creates a hazard of injecting paint through
the skin.
Airless Spray

39. Airless Spray

40. Airless Spray
• In large-scale operations the paint is drawn from
barrels through hoses, often over a considerable
distance.
• To facilitate the application and pumping, the paint
can be heated, often as high as 75 C, and the lines are
‘traced’ with electrical heating tape.
• Where two-pack paints are employed, the
components can be mixed prior to spraying.
• Such mixtures usually have a finite lifetime (the ‘pot
life’) before they become too viscous to handle and
would eventually gel with disastrous implications for
the equipment.
• It is therefore essential to adopt a procedure that
ensures use of all the mixed material in a
predetermined time.

41. Electrostatic
Spraying

42. Electrostatic Spraying
1. Pneumatic and
2. Airless spray application
always suffer from ‘overspray’.
• A proportion of the atomised paint particles miss the substrate,
especially in windy conditions.
• Overspray not only represents a waste of material, but also can
be a considerable nuisance since it is sometimes carried for
miles.
• Gratuitous repainting is not always welcomed by the owners of
cars parked in the vicinity.
• Electrostatic painting was introduced in the 1940s to minimise
the problem of overspray.

43. Electrostatic
application:
• Paint particles are
negatively charged
when passing
through a paint
gun.
• The paint is
attracted to the
object being painted
which is positively
charged and
grounded.
• Sparking hazards
are of particular
concern.
- +
Electrostatic spray

44. Electrostatic spray
 Most common method
 High voltage, low
amperage field
generated by spray
gun imparts
electrostatic charge
on powder
 Charged powder
sticks to grounded
substrate
 Object is heated in
oven to cure, powder
flows to form a
continuous film

45. Curing
(Infrared oven)
Spraying (charged particles)

46. Spraying
• Electrostatic conditions can be used with pneumatic
or airless spray, but are even more important in the
case of powder coating where a charge is required to
enable the particles to adhere to the substrate until
fused in the oven.
• Powder particles are charged by corona discharge or
by tribo-charging in their passage through a gun
lined, for example, with polytetrafluoroethylene.
• The composition of the powder particle surface is
critical to the efficiency of tribo-charging and
additives are often incorporated.
• Hindered amines can be used in the original
formulation, or alumina can be post blended into the
powder.

47. Electrodeposition

48. Electrodeposition
• Electrodeposition is an
immersion painting process in
which charged paint particles
are attracted to an oppositely
charged metallic surface.
• As the paint is deposited, it
forms a finish which begins to
insulate the metal from the
surrounding charged solution.
• Deposition continues until the
coating thickness becomes
sufficient to form a barrier
against further paint attraction.

49. Electrocoating or E-coat
The Electrocoating Process...
• Precipitation of paint particles onto a metal substrate
• Highly efficient and automated process
• Paint deposition is regulated by voltage
• Coating can be either anodic or cathodic
• Thermoset curing
PPG

50. E-Coat: Anodic Coatings
Epoxies (cure >80 °C)
Acrylics (cure > 150 °C)
Acid-modified polybutadiene
Butylated-formaldehyde-melamine (150 °C)
Anode has a positive charge that attracts the negatively
charged polymers
CO2H
HO2C
KOH
O
O
O
O

51. Anodic electrocoating
• In anodic electrocoating, the part to be coated is
the anode with a positive electrical charge which
attracts negatively charged paint particles in the
paint bath.
• During the anodic process, small amounts of
metal ions migrate into the paint film which limit
the performance properties of anodic systems.
• The main use for anodic products is interior or
moderately exterior environments.
• Anodic coatings are economical systems that

52. Epoxies (cure >190 °C)
Acrylics (cure > 190 °C)
E-Coat: Cathodic Coatings
Cathode has a negative charge that attracts the positively
charged polymers

53. Cathodic electrocoating
• In cathodic electrocoating, the product has a negative
charge, attracting the positively charged paint particles.
• Cathodic electrocoat applies a negative electrical charge
to the metal part which attracts positively charged paint
particles.
• Reversing the polarities used in the anodic process
significantly reduces the amount of iron entering the
cured paint film and improves the cathodic properties.
• Cathodic coatings are high-performance coatings with
excellent corrosion resistance that can be formulated for
exterior durability.

54. • Corrosion resistant cationic epoxies
• High edge coverage for sharp objects
• Solvent free anodic products
• Cathodic acrylics with corrosion resistance and excellent exterior durability
• Near 100% transfer efficiency operation
• Two-coat Electrocoat for severe environments
• Heavy-Metal free formulations
• HAPs free formulations
• Electrodepositable Photoresists
Agriculture Equipment, Appliance,
Automotive, Fasteners, Metal Office
Furniture, Printed Circuit Boards,
Structural Steel, Wheels, Railway

56. Automobile Painting

57. •Reading Assignment
•Pages 354-359
•Marrion, A. Ed., The
Chemistry and Physics of
Coatings, Second Edition,
The Royal Society of
Chemistry, 2004