Surface Preparation and Primer Selection by Substrate
Most coating failures start at the substrate interface. The preparation and the primer are chosen together, by material.
Plaster and render
- New: allow to cure (2–4 weeks), brush down, fill and sand. Prime with an alkali-resisting primer or a thinned first coat of emulsion.
- Old/painted: wash, scrape flaking areas, spot-prime bare patches, sand glossy areas.
Concrete and masonry
- Remove laitance and form-release agent (acid etch or grind for floors). Fill blowholes.
- Prime with a penetrating masonry sealer; for floors, an epoxy primer. Test pH and moisture on power-floated slabs.
Timber
- Sand with the grain, spot-treat knots with knotting solution, fill nail holes after priming.
- Prime with an aluminium wood primer or acrylic primer-undercoat; end grain gets extra.
Ferrous metal
| Prep standard | Result |
|---|---|
| St 2 / St 3 | Hand/power tool cleaning — light duty only |
| Sa 2½ | Near-white blast — standard for durable systems |
| Sa 3 | White metal blast — immersion, severe service |
Prime within the recoat window (before flash rust) with a zinc-rich or zinc-phosphate epoxy primer.
Galvanised and non-ferrous
Degrease, then either weather for 6+ months, sweep-blast, or apply a mordant/T-wash, and prime with a dedicated etch primer or a primer proven over zinc. Never use a standard alkyd primer directly on galv — it saponifies and peels.
EngEst Pro’s Painting Calculator sets the preparation grade and primer per surface type in your schedule, and carries the prep labour, abrasives, and primer quantities into the BOM alongside the finish coats.
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