Which primer do I need for which surface?

The primer aisle is organised by product name, which is why people buy the wrong one. Reorganise it by problem and it becomes simple: there are only about five things that can be wrong with a surface, each has a class of primer that addresses it, and a surface can have more than one problem at once — a bare softwood window has porosity, a resin bleed and end grain to deal with, and needs three answers, not one.

Step by step

1. Ask first whether the surface drinks paint.New plaster, new plasterboard, bare timber, MDF, unpainted render and concrete all absorb. Left unsealed they suck the binder out of the first coat, leaving a weak, patchy, powdery film. The answer is a sealing primer: a thinned mist coat on plaster, a drywall primer-sealer on board, wood primer on timber, an MDF sealer on board edges.
2. Ask whether anything will stick to it.Cured gloss, melamine, laminate, tile, uPVC, rigid plastic, glazed surfaces and previously waxed or polished wood have no porosity and nothing for paint to grip. The answer is an adhesion or bonding primer, plus a mechanical key from abrading, plus a serious degrease. All three, not one.
3. Ask whether the substrate is chemically hostile.New cement render, concrete, mortar and fresh plaster are alkaline. Free lime attacks an oil-based film and destroys it — the film breaks down and comes away as a soapy mess, which is called saponification. The answer is an alkali-resisting primer. Bare ferrous metal is hostile in a different way, and needs a rust-inhibiting primer.
4. Ask whether something will come through the paint.Water stains, nicotine, soot, smoke damage, felt tip, biro, crayon, bitumen, timber tannin and resinous knots all migrate through a water-based film and reappear at the surface. The answer is a barrier: a solvent-based or shellac-based stain block for stains, and shellac knotting solution on knots.
5. Ask whether the surface is loose or powdery.Chalking exterior masonry, distempered ceilings, old flaking limewash and dusty render will take the new paint off with them. The answer is a stabilising solution or masonry stabiliser, applied after brushing off everything loose, which soaks in and binds what remains.
6. Count the problems, not the surfaces.A bare softwood window frame absorbs, has knots that will bleed, and has end grain that absorbs far more than the face. That is three jobs: knotting on the knots, extra primer into the ends and joints, and a full priming coat everywhere. One universal primer does not do all three.
7. Then check whether you need a primer at all.Sound, clean, matt existing paint of a compatible type needs no primer — it needs washing and a light abrade. A primer is a cost and a coat, and applying one where nothing is wrong buys nothing.

Tips

• PVA is a bonding agent for plaster and tile adhesive, not a paint primer. Emulsion over a PVA-sealed wall has a long history of peeling off, because the PVA re-softens with moisture.
• Most primers have an overcoating window and some are quite specific about it. Check both the minimum and the maximum before you plan the day.
• When you genuinely cannot identify a surface, prime a test patch, leave it a day, and try to lift it with a fingernail and a piece of tape. That test takes ten minutes and settles the argument.

Common mistakes

• Buying an all-purpose primer for a difficult substrate — Universal primers are aimed at ordinary porous surfaces. On melamine, tile, galvanised steel or a resinous knot they are simply the wrong chemistry and fail.
• Using an oil-based primer on new render or fresh plaster — The alkalinity attacks the oil binder. The film breaks down and comes away, and the only fix is to remove it and start again with an alkali-resisting primer.

Questions people ask