Surface Preparation and Substrate Assessment
Preparation decides whether a paint film survives. The exam tests whether you can judge a substrate - plaster, concrete, brick, timber, steel, galvanised sheet or an old painted surface - and prescribe the correct preparation and primer.
Assessment checks
- Moisture: masonry should read below roughly 12–15% on a moisture meter, and timber lower still. Damp walls blister and peel.
- Alkalinity: new lime-bearing plaster is alkaline; a pH test above about 9 means alkali-resisting primer is required, not direct emulsion.
- Efflorescence: white salt bloom is brushed off dry and the source of damp traced; washing pushes salts back in.
- Timber: resinous knots need shellac knotting, or resin bleeds through; nail heads are punched and primed.
- Steel: rust is scraped or wire-brushed to bare metal and covered at once with a rust-inhibitive primer.
- Old paint: chalking, flaking and gloss are deglossed and feathered; unstable coats are stripped.
Sequence
Clean → repair cracks and holes → sand → dust off → prime → fill → sand → undercoat.
Worked example
A 12 m² internal wall reads pH 10 and 16% moisture. Reasoning: both readings fail, so no coating will hold. Correct instruction: find and cure the damp, let the plaster dry to below 12%, then apply one coat of alkali-resisting primer before two coats of emulsion. Painting now would guarantee blistering within one monsoon.
Traps that cost marks
Priming over damp; sanding old lead paint dry; skipping knotting; filling over dust; using emulsion as a primer on bare plaster; ignoring a hollow, blown patch.