Surfaces, Substrates and Surface Preparation
Surfaces, Substrates and Surface Preparation
This domain tests whether you can identify a substrate, judge its condition, and prepare it so the coating can bond. Most coating failures trace back to preparation, so exam questions reward specific preparation steps.
Substrates and key preparation
- Wood: sand with the grain, fill defects, and knot with shellac or knotting before priming. Moisture content should be near 10–15%.
- Ferrous metal: remove mill scale, rust and grease. ISO 8501-1 grades run from St 2/St 3 hand and power tool cleaning to Sa 1, Sa 2, Sa 2½ and Sa 3 blast cleaning. Sa 2½ is near-white blast cleaning and is a common specification.
- Galvanised steel: degrease and either sweep-blast or use a zinc-compatible etch primer. Do not apply a plain alkyd directly.
- Masonry, concrete and plaster: allow to dry, remove efflorescence, treat salts, and prime with an alkali-resistant primer. New plaster may need a mist coat.
- Previously painted surfaces: wash, degrease, sand to key the surface, and remove loose or flaking paint.
Worked example
New galvanised steel is to be painted. The surface has mill oil and a smooth zinc spangle. The correct sequence is solvent degrease, fresh-water wash, sweep blast or abrade, then a zinc-compatible primer. The reasoning is that the oil blocks adhesion and the smooth zinc surface gives no mechanical key.
Traps
- Painting damp masonry and trapping moisture, causing blistering and efflorescence.
- Skipping knotting and letting resin bleed through the topcoat.
- Abrading galvanised steel with a steel wire brush that leaves iron particles and starts corrosion.
- Failing to remove soluble salts before coating.