Do You Always Need a Primer? A Decorator's Guide to Priming Walls

Ask ten decorators about priming and you will get ten slightly different answers. Primer is one of the most important — and most frequently skipped — steps in professional painting and decorating. Getting it right makes the difference between a result that lasts a decade and one that fails within two years. Here is an authoritative guide from decorators who prime correctly on every project across Hampstead, Highgate, and North London.

What Does Primer Actually Do?

Primer serves several distinct functions depending on the context:

• Adhesion: Primers are specifically formulated to bond to bare or difficult surfaces — raw plaster, bare timber, metal, previously painted surfaces with adhesion problems — and provide a stable base for subsequent coats.
• Sealing: Primers seal porous surfaces, preventing the finish coat from soaking in unevenly. This is critical on bare plaster, which without sealing will absorb emulsion rapidly and leave a patchy, uneven result.
• Stain blocking: Specialist stain-blocking primers prevent water stains, smoke marks, tannin bleed from timber, and other contaminants from bleeding through subsequent coats of emulsion or eggshell.
• Reducing the coat count: On very porous or absorbent surfaces, a primer can reduce the number of finish coats needed by filling the porosity of the substrate, saving time and cost overall.

When Primer Is Essential

New Plaster

This is the most important and most frequently encountered situation in North London, where renovation and plasterwork are constant. New plaster is extremely porous and alkaline. Applying neat emulsion directly to new plaster results in patchy, uneven coverage and poor adhesion. The correct approach is a mist coat — neat emulsion diluted 80:20 with water — as the first coat, sealing the plaster before full-strength emulsion is applied.

Bare Timber

Bare wood must always be primed before painting. Timber contains moisture, natural oils, and in softwoods, knots that bleed resin — all of which will prevent paint adhesion or bleed through finish coats if not sealed properly. Use a wood primer on most bare timber; use shellac-based knotting compound on resinous knots before the wood primer is applied.

Bare Metal and Ironwork

Victorian ironwork railings and window furniture throughout Hampstead and Highgate require rust-inhibiting primer before any topcoat. Without it, paint will fail rapidly and rust will spread beneath it.

Stain-Blocking Situations

Any surface with water stains, nicotine, mould, or other contamination requires a stain-blocking primer — typically a shellac-based product such as Zinsser BIN. Painting over stains without a stain blocker results in the contamination bleeding through even multiple coats of emulsion.

When Primer May Be Optional

On previously painted surfaces in good condition — sound, clean, and compatible with the new paint — a separate primer is often not necessary. If repainting a well-prepared wall with a compatible emulsion from the same paint system, the existing paint acts as its own primer. The same applies to woodwork: if repainting eggshell over eggshell on skirting boards that are in sound condition and have been lightly sanded, a separate primer is not always required.

However, if you are changing from an oil-based to a water-based paint system (or vice versa), or if there is any doubt about the adhesion of the existing surface, primer is always the safer choice.

Choosing the Right Primer

• New plaster: diluted emulsion mist coat or specialist plaster primer
• Bare softwood: oil-based or water-based wood primer
• Bare hardwood or MDF: appropriate wood primer or specialist MDF primer
• Metal and ironwork: rust-inhibiting metal primer
• Stains and contamination: shellac-based stain blocker (Zinsser BIN)
• Adhesion problems or system change: adhesion primer

Call Hampstead Painting Company on 020 3874 2670 to discuss your painting project with an experienced decorator.