What is primer?
Primer is the first coat of a paint, adhesive or sealing system, applied to obtain adhesion to the substrate and to give that substrate an even absorbency. The primer is not a finish in itself: it is the link between a substrate that will not hold paint on its own and the coat that does the visible and protective work.
The terminology gets muddled in practice. Primer, undercoat, sealer and bonding primer are used interchangeably, although they do not do the same job.
What a primer does
| Function | Explanation |
|---|---|
| Adhesion | Forms a chemical or mechanical bond with a smooth or difficult substrate |
| Controlling absorbency | A porous substrate draws the binder out of the finish coat; the primer stops that |
| Sealing | Blocks bleeding substances: tannin from oak, resin from knots, nicotine, soot, water stains |
| Corrosion protection | On steel, the primer carries active pigments that inhibit rusting |
| Colour base | Gives the finish coat an even base colour, so fewer coats are needed |
Choosing a primer by substrate
- New, bare exterior timber — a well-penetrating wood primer; seal knots first
- Plasterboard and new plaster — a sealer that evens out absorbency
- Calcium silicate block, concrete, cement-based substrates — a deeply penetrating, alkali-resistant primer
- Steel — a corrosion-inhibiting metal primer on de-rusted, degreased, clean steel
- Galvanised steel, aluminium, hard plastics, tiles — a dedicated bonding primer; ordinary undercoat will not hold
- Stains and bleed-through — a blocking primer, usually alcohol- or water-based with a stain-blocking binder
Practical rules
- The substrate decides everything. Dust-free, grease-free, dry and sound; otherwise even the best primer will not hold.
- Respect the overcoating time on the tin. Overcoating too early traps solvent; overcoating too late can reduce adhesion on products with a limited recoat window.
- Primers are usually not UV- or weather-resistant. Do not leave a primed surface outdoors unfinished for weeks.
- Stay within one system. Primer and topcoat from the same manufacturer and the same range give the fewest surprises.
- In doubt? Lay a trial patch of half a square metre and test the adhesion after curing with a tape test.
Related terms
- Primer standing time
- Red lead primer
- Adhesion
- Painting
- Structural steel sections
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