Surface Preparation and Substrate Treatment for Painting
Surface preparation decides whether the job survives its warranty. The exam tests substrate-specific preparation for plaster, wood and metal, and the moisture and alkalinity limits behind each step.
Plaster, masonry and concrete
- Let new plaster cure (about 28 days) and dry; check moisture below roughly 12-15 percent with a meter before painting.
- Brush off loose particles; remove efflorescence by dry brushing and washing; treat mould with fungicidal wash and allow to dry.
- Fresh alkaline plaster needs an alkali-resisting primer. Oil or enamel paint straight on new cement or lime plaster saponifies and peels.
Wood
- Moisture content about 10-12 percent for interior joinery.
- Sand along the grain; cut back raised grain with 120 then 180 grit.
- Knots: seal with two coats of shellac knotting to stop resin bleed.
- Prime first, then fill nail holes and cracks with putty or wood filler; putty on bare, dusty wood stains and cracks. Sand filler flush.
Ferrous metal
- Remove rust, scale and flaking paint to bare metal (hand St 3, blast Sa 2 1/2).
- Degrease, then prime the same day, typically within four hours of cleaning, with red oxide, zinc phosphate or zinc chromate primer.
- Galvanised steel and aluminium need an etch or mordant wash and a compatible primer, otherwise the finish will not adhere.
Worked example
A three-month-old cement-plastered bedroom wall shows white fluffy patches. Correct sequence: brush the deposit off dry, wash, apply fungicidal wash if mould is present, dry to below 12-15 percent moisture, apply alkali-resisting primer, one coat putty, sand, then two coats acrylic emulsion. Painting it directly with synthetic enamel traps alkali and gives saponification, blistering and early peeling.
Traps
Painting a damp wall; priming over rust; putty over bare dusty plaster; skipping the etch primer on galvanised sheet.