Surface Preparation and Substrate Repair
Surface preparation is the single largest cause of coating failure, and the exam tests the sequence as much as the products.
The sequence
- Identify the substrate, then clean, repair, sand, and prime.
- Wash with TSP substitute or a degreaser, rinse, and dull gloss with 120-150 grit.
- Drywall: square out holes, add a backer, tape, apply three thin coats of compound, feather 6-8 in, sand with 120-150 grit.
- Wood: set nails, fill with wood filler, seal knots with shellac-based knot sealer, sand 80 then 120-150 grit with the grain.
- Metal: remove rust to bare metal and prime with a rust-inhibitive primer inside the recoat window.
- Masonry: remove efflorescence, fill voids, and use an alkali-resistant masonry primer.
Primer matching
Bare drywall takes PVA or drywall primer; bare wood takes wood primer; galvanized metal takes a galvanized-metal primer; stains and knots take shellac or an oil stain blocker.
Worked example: a 4 in hole in drywall. Cut it to a square, install a backer, cut the patch, tape, apply three coats of compound with drying between, sand, then spot prime. Skip the spot prime and the patch flashes, because the compound absorbs differently than the paper face, so the topcoat shows a blotch.
Traps
- Painting over chalky, dirty, or glossy surfaces.
- Not spot-priming repairs.
- Dry-sanding lead paint.
- Using the wrong caulk, such as a rigid caulk where movement is expected.