Primer (paint)
A primer is the first paint coat that bonds to the substrate, seals it, and gives topcoats something to grip; the right primer is matched to the surface, not generic.
Ask Chalkline about this →A primer is the first paint coat applied to a prepared substrate. It bonds to the surface, seals it, and gives the topcoats something to grip. The right primer is matched to the substrate and the problem, not a generic “undercoat”, and choosing it wrong is a common reason a paint job fails early.
A primer does one or more jobs depending on the surface:
- Adhesion: keys the topcoat to a difficult surface (glossy, dense, or non-porous).
- Sealing: stops a porous substrate (new render, plasterboard, bare timber) sucking the topcoat in unevenly, and locks down chalky or powdery surfaces.
- Blocking: stop-stain and alkali-resistant primers block bleed-through (tannin from timber, water stains) or the high alkalinity of fresh render and concrete that would otherwise burn an ordinary topcoat.
- Protection: metal primers provide the corrosion-inhibiting first layer (matched to the exposure).
“Primer”, “sealer”, and “undercoat” overlap in usage, but the point is the same: it is the coat that does the bonding and sealing so the finish coats can do the looking. It only works on a properly prepared surface.
For a builder the practical points are to match the primer to the substrate (alkali-resistant on fresh render, stain-block over timber knots and water marks, a metal primer on steel for the exposure), not to skip it on porous or difficult surfaces to save a coat, and to let it cure before topcoating. Topcoats applied straight onto an unprimed or wrongly-primed surface are exactly the jobs that peel, flash, or bleed within a year.
Also known as: Sealer, undercoat, primer-sealer.
Category: Finishes / Paint.
Related
See also
References
- Surface preparation (Chalkline) (verified 2026-06-04)
Last updated: 2026-06-04. Verified: 2026-06-04. Quarterly review for currency.