This document provides information on various surface defects and how to repair them. It discusses removing rust by hand or power tools, treating friable surfaces before painting, removing mould growth, repairing defective puttying and plasterboard, dealing with efflorescence on cement, repairing damaged rendering, and properly applying caulk. The key is to prepare surfaces thoroughly, use the right tools for each job, and avoid over-polishing or damaging the surface further during repairs.

1. © City of Glasgow College
Charity Number SC0 36198
Surface Defects

2. You can clean off rust by hand, using a
wire
brush, paint scraper, chipping hammer
or
abrasives – or a combination of all of
these!
• Remove traces of oil and grease.
• Scrape off all loose rust.
• Use a chipping hammer around
rusted nails, etc.
• Use a wire brush, but avoid
burnishing.
• Finish off with a rough aluminium
oxide abrasive.
What is burnishing?
Corrosion or rust: removing by hand

3. It is quicker and more effective to
use
power tools to remove rust – but
take
care not to over-polish the surface.
• Remove traces of oil and
grease.
• Scrape off all loose rust.
• Use a needle gun to remove
rust around corroded nuts, etc.
• Select appropriate method:
sander, rotary wire brush,
angle grinder.
Why is over-polishing a problem?
Corrosion or rust: removing with power tools

4. Paint can be removed in a
number of ways:
• with an LPG burning off
torch
• with a hot air stripper
• with chemical paint
remover.
What are the advantages and
disadvantages of each
method?
Painted surfaces in poor condition

5. Abrading surfaces
Abrading means wearing away the top
layer
by rubbing. This provides a key for the
coating or covering to be applied to, and
gives a good quality finish. Abrasive
materials need to be:
• not too rough – this can leave
scratches
• not too fine – this may be ineffective
• of good quality – cheap ones don’t
work well!
What are wet and dry abrasives? What
are
dry abrasives?

6. Mechanical sanders can be
quicker and more effective
than manual sanding
techniques, but may only be
suitable for certain areas –
and can create a lot of dust.
When is it best to use a
heavy duty sander, and when
a rotary sander?
Mechanical sanding

7. A friable surface is one that
crumbles away easily when
you rub your hand over it, such
as weathered brickwork. If
paint is applied to a friable
surface, it won’t last very long
as it will come off as the
surface crumbles.
How should you treat a friable
surface before painting it?
Friable surfaces

8. To remove mould growth:
• Wet the mould.
• Remove heavy patches
with a scraper or brush.
• Apply a fungicidal wash.
• Leave for a week and re-
treat if necessary.
Why do you need to wet the
mould before removing it?
Mould

9. Defective putty
If you notice a gap between
the old putty and the glass,
you must seal it completely by
forcing in linseed oil putty
using a putty knife.
What should you do if any
bare timber is exposed?

10. Apply a hard waterproof
coating to plasterboard before
applying surface coverings
like wallpaper. This will stop
the covering bonding with the
surface, and prevent damage if
the wallpaper is removed later.
How is plasterboard
constructed?
Damage to plasterboard

11. Moisture penetrates cement, dissolving
some of the lime, creating calcium
hydroxide which rises to the surface as
a white patch. Remove any
efflorescence with a stiff fibre brush or
wire brush before decorating.
Why should you not try to wash off
efflorescence?
Efflorescence

12. • Rake out any loose coatings and rendering.
• Undercut the crack.
• Point up the crack with mortar.
What tools are needed to repair this defect?
Repairing damaged rendering

13. Caulk is a waterproof filler and sealant,
used
in cracks and gaps. Mastic is one sort of
caulk.
To apply caulk correctly:
• apply the caulk, usually with a gun
• wipe across the bead with a wet finger
• remove any surplus with a filling knife
• sponge off any remaining material.
Why is one of the applications on the
right
incorrect?
Caulking