When to Use a Primer: A Complete Guide for London Homeowners

What a Primer Actually Does

A primer is not simply a cheap first coat of paint. It is a chemically distinct product designed to perform a specific function: to create the ideal interface between the substrate (the surface you are painting) and the topcoat (the paint you want to see at the end). When a decorator says a primer is needed, they are not padding out the schedule — they are protecting the quality of the finished job and the longevity of the system.

The functions a primer performs include sealing a porous surface so the topcoat does not sink in unevenly, promoting adhesion on surfaces that paint would not otherwise bond to, preventing staining from bleeding through the topcoat, and building a stable base on new or problematic substrates.

Understanding when a primer is genuinely needed — and when it can reasonably be skipped — will help London homeowners have more informed conversations with their decorators.

New Plaster: Never Skip This Step

Freshly plastered walls are one of the most critical applications for a primer. New plaster is highly alkaline (pH 12 or above when fresh) and extremely absorbent. If a standard emulsion is applied directly to new plaster, two things happen: the plaster draws the water out of the paint almost instantly, preventing proper film formation, and the resulting dry surface has poor adhesion and patchy colour.

The correct approach for new plaster is a mist coat: a heavily diluted emulsion (typically 20 to 30% water added to the paint) or a purpose-made plaster primer such as Dulux Trade Plaster Sealer or Zinsser Bulls Eye 1-2-3. This allows the paint to penetrate slightly into the plaster surface and cure properly, creating a sealed, stable base for the subsequent topcoats.

Never use neat emulsion on fresh plaster. It is one of the most common mistakes made in newly built or newly plastered London properties.

Bare Wood: The Case for Primer

Bare or stripped-back timber requires a primer for several reasons. Wood is a variable substrate — it has alternating bands of earlywood and latewood with different densities and absorption rates. Without a primer, the topcoat will sink unevenly into these bands, creating a patchy, uneven finish.

More importantly, wood contains resins and natural tannins that can bleed through a topcoat if not sealed. This is particularly pronounced in knots, where resin concentration is highest. On softwood (which is the most common timber in London joinery — pine skirtings, MDF architraves, etc.), knots must be treated with a knotting solution (traditional shellac-based or modern synthetic) before any primer is applied.

Recommended primers for wood:

• Solvent-based wood primer (such as Dulux Trade Wood Primer or Johnstone's Trade Undercoat) — the traditional choice, excellent penetration and adhesion, but slow drying and higher VOC
• Water-based acrylic primer — faster drying, lower odour, compatible with water-based topcoats. Products such as Dulux Trade Waterborne Primer, Teknos Futura Aqua Primer, or Zinsser Bulls Eye 1-2-3 all perform well
• Stain-blocking primer (Zinsser BIN shellac-based primer) — for heavily knotty or resinous timber, or for covering water stains, smoke damage, or tannin bleed

Bare Metal: Adhesion and Rust Prevention

Metal surfaces — whether mild steel railings, radiators, cast iron, or aluminium — require a primer that is specifically designed for metal adhesion and, in the case of ferrous metals, rust prevention.

Applying standard emulsion or even standard solvent-based paint directly to bare metal without a primer will result in poor adhesion and rapid corrosion. The primer must do two things: create a mechanical key and a chemical bond with the metal surface, and provide a barrier against moisture ingress.

Types of metal primer:

• Red oxide primer — the traditional choice for ferrous metals. Excellent corrosion inhibition, a little old-fashioned but still widely used on structural steelwork and gates
• Zinc phosphate primer — more modern, provides excellent corrosion resistance through the zinc phosphate's reaction with the metal surface
• Universal adhesion primer — products such as Zinsser Bulls Eye 1-2-3 or Rustins Stain Block can be used on metal and provide a good base for topcoats where a specialist metal primer is not required

All metal must be thoroughly cleaned and degreased before priming. Any rust should be treated with a rust converter or removed mechanically before the primer is applied.

PVC and Plastic: An Often-Overlooked Surface

PVC window frames and plastic surfaces are notoriously difficult to paint without the correct preparation. PVC has very low surface energy, which means paint beads on it like water on a waxed car rather than bonding to the surface. Without the correct primer, paint on PVC will peel within weeks.

The correct approach is either:

• A dedicated PVC primer (Thorndown PEELaway or Zinsser Bulls Eye 1-2-3 perform well on PVC when applied correctly)
• Light mechanical abrasion of the surface with fine-grit paper followed by a solvent wipe and then a primer

PVC painting is a valid and cost-effective alternative to replacement for window frames and doors, but only when properly primed and topcoated.

Previously Painted Surfaces: When Is a Primer Still Needed?

If a surface has been previously painted and is in sound condition — no flaking, no staining, no significant colour change intended — a primer is often not needed. Two coats of a quality topcoat over a lightly sanded and degreased existing finish will give a durable, attractive result.

However, a primer or stain-blocking product is needed in the following situations:

• Changing from dark to light — a stain-blocking primer such as Zinsser BIN or Bulls Eye 1-2-3 will significantly reduce the number of topcoats needed to achieve full opacity
• Water staining — water marks from leaks or condensation will bleed through standard paint. A shellac or oil-based stain blocker is required
• Nicotine or smoke damage — heavy nicotine staining will bleed through multiple coats of emulsion. Zinsser BIN is the industry standard solution
• Over a glossy surface — if the existing finish is a high-gloss that has not been sanded, adhesion primer improves the bond of the new topcoat

The Cost of Skipping Primer

A primer coat adds cost and time to a painting project. But the cost of a failed paint job — topcoat peeling from bare wood, emulsion sinking into new plaster, topcoat yellowing from tannin bleed — is always higher. A professional London decorator will always specify the appropriate primer system as part of a properly costed proposal. If a quote omits primer coats on surfaces that clearly require them, it is worth asking why.

Belgravia Painters specifies and applies the correct primer systems on every project we undertake across London. Contact us for a free assessment and quote.