Prepare Surfaces and Substrates for Painting
This domain tests surface preparation, the step that decides whether the coating will adhere. Expect concrete and masonry, wood, ferrous and galvanised metal, drywall and previously painted surfaces. The exam looks for moisture and pH checks, removal of contaminants, correct profile, patching, and primer selection.
Core rules
- Concrete and masonry: cure properly, check moisture (commonly 12–15% or lower with a meter) and pH (around 10 or below before alkali-resistant primer). Remove form oil, grease, dust, efflorescence and loose material; rinse and neutralise as required.
- Wood: moisture around 12–15%; sand with the grain, remove mill glaze, seal knots with shellac or a knot sealer, fill defects with a compatible filler, and dust off.
- Steel: remove rust, mill scale, oil and salts; abrade to the specified cleanliness and profile, then prime the same day. Galvanised steel needs weathering, sweep blasting or an etching primer.
- Drywall: fix joints, embed tape, sand, remove dust, spot-prime repairs so the finish does not flash.
- Feather edges of old paint and test for lead before dry sanding in pre-1978 buildings.
Worked example
A concrete wall shows white powder and peeling paint. Sanding and repainting will fail. The powder is efflorescence, indicating moisture movement and salts. The repair sequence is to find and stop the water source, dry the wall, brush off salts, wash and neutralise, check pH and moisture, apply an alkali-resistant primer, patch and then finish. The reasoning is that coating over salt or damp concrete traps the problem.
Traps
- Painting damp concrete or fresh plaster.
- Using putty over dust or oil.
- Skipping primer on bare steel or galvanised sheet.
- Sanding lead paint dry without controls.