Primers Explained: When and Why London Painters Use Them
A clear, practical guide to primers for London homeowners and property managers — what primers actually do, when you genuinely need one, the difference between new plaster mist coats, bare timber primers, stain-blocking shellac, and metal primers, MDF edge sealing, and when skipping primer causes costly failures.
What a Primer Actually Does
The word "primer" is used loosely in the painting trade to mean any coat that goes on before the finish. But primers work through several distinct mechanisms, and understanding which mechanism you need for a given surface determines which product is appropriate. Using the wrong primer — or skipping it entirely — is one of the most common causes of paint failures that we are called in to rectify.
There are four things a primer may need to do:
- Adhesion. Provide a surface that the topcoat can grip, particularly over smooth, non-porous, or contaminated substrates
- Sealing. Reduce the porosity of an absorbent substrate so that the topcoat does not sink in unevenly
- Blocking. Prevent contaminants in the substrate (stains, tannins, resin, soluble salts) from bleeding through into the topcoat
- Sacrificial build. Provide a layer of material that can be sanded back to achieve a flat, even surface before the finish coats go on
Many products do more than one of these things simultaneously. But the primary need determines the specification.
New Plaster: The Mist Coat
Fresh plaster is one of the most common surfaces encountered in London renovation work, and it is one that is most frequently mishandled. The mistake is simple: applying standard emulsion directly to new plaster.
Why it fails. New plaster is highly alkaline and extremely porous. When standard emulsion is applied to it, the plaster draws moisture from the paint so rapidly that the paint cannot form a continuous film. The result is a chalky, powdery surface that can be rubbed off with a finger, and which will not support the finish coat.
The mist coat. The correct approach for new plaster is to apply a mist coat first — a heavily diluted emulsion (typically sixty percent paint to forty percent water, though the ratio varies with the dryness and absorbency of the plaster) that penetrates the surface and stabilises it. The mist coat should be allowed to dry completely — at least twenty-four hours in most conditions, longer in cold or humid weather — before the first full-strength coat is applied.
Plaster colour. New plaster changes colour as it dries, moving from a dark, wet brown/orange to a pale, uniform cream. The plaster must be fully dry — fully pale in colour, with no darker patches — before any coating is applied. Painting over damp plaster guarantees failure, regardless of the product used.
Purpose-made alternatives. Products like Zinsser Gardz and Johnstone's Flat Matt Sealer are designed as single-product solutions for new plaster that combine the sealing function of a mist coat with a tougher base for the finish coats. These are particularly useful for patchy or problem plaster surfaces.
Bare Timber: Matching Primer to Purpose
Bare timber is encountered on every interior joinery painting project — new timber, stripped timber, or spot-prepared areas where old paint has been removed. The appropriate primer depends on the type of timber and its condition.
General Timber Primer
For general bare timber — softwood skirtings, architraves, door frames, and door faces that are free of major defects — a standard acrylic or alkyd wood primer provides adequate adhesion and sealing. Water-based acrylic primers are fast-drying and easy to sand; traditional alkyd (oil-based) primers penetrate more deeply and are often preferred for exterior timber where a more substantial base is needed.
Resinous Timber and Knots
The critical exception is resinous timber. Many softwoods — particularly pine, which is the most common timber in London's Victorian and Georgian joinery — contain localised pockets of resin concentrated at knots. Resin is not soluble in water-based products, and it bleeds through water-based primer and topcoats, creating yellow/brown rings that are visible through the finish.
Knotting solution. The traditional treatment for resinous knots is shellac knotting solution — a shellac-based sealer applied directly to the knot before priming. Shellac forms an effective barrier against resin bleed and is compatible with both water-based and oil-based systems applied over it. The conventional product is Rustins Knotting Solution, and it should be applied before any primer to all visible knots.
Shellac BIN. Zinsser BIN is a shellac-based primer-sealer that provides both stain-blocking and adhesion properties in a single product. For heavily resinous timber or timber with multiple knots, BIN can be applied as an all-over coat in lieu of knotting plus primer. It is fast-drying (touch-dry in twenty minutes) and provides an excellent base for any topcoat.
Previously Painted Surfaces: When to Prime, When to Skip
The decision on whether to prime over previously painted surfaces is one of the most common judgement calls in interior decoration, and the right answer depends on the condition of the existing surface.
When spot priming is sufficient. On a surface in generally good condition — sound, well-adhered paint with only minor areas of chips, scuffs, or repairs — spot priming the repaired areas is appropriate. Apply a compatible primer to the bare patches, allow to dry, lightly sand, and then apply the full finish coats to the whole surface. The existing paint provides adequate adhesion for the topcoat over the undamaged areas.
When full repriming is necessary. On a surface in poor condition — widespread adhesion failure, peeling, a surface that has been stripped back significantly, or where a dramatically different paint type is being applied — a full reprime of the whole surface is the right approach. Applying a new topcoat over a failing old paint system does not fix the underlying problem; it compounds it.
Sheen compatibility. Applying a matt or mid-sheen topcoat over a high-gloss existing surface is an adhesion risk. Gloss surfaces are non-porous and non-porous surfaces do not hold new paint well without mechanical preparation (sanding) or a bonding primer. Always sand high-gloss surfaces before applying new coats, or apply a purpose-made adhesion primer.
Stain Blocking: Shellac BIN, Oil-Based, and Specialist Products
Stain blocking is a distinct primer function, separate from adhesion and sealing, and it is where the choice of product matters most.
Water stains. Water stains from roof leaks, burst pipes, or condensation are soluble in water-based primers and topcoats, meaning they will bleed through into the finish and remain visible even after multiple coats. A shellac-based primer (BIN) or a solvent-based stain blocker is required. After the stain-blocking coat has dried and been confirmed as effective (the stain does not bleed through), standard water-based topcoats can be applied.
Smoke and fire damage. Soot and smoke staining from fire or heavy smoking is one of the most challenging stain-blocking applications. Multiple coats of shellac BIN are typically required, with confirmation that no staining is visible before topcoating. For severe smoke damage, oil-based stain-blocking primers (such as Zinsser Cover Stain) may be more effective than shellac alone.
Tannin staining. Timber species with high tannin content — oak, western red cedar, and certain tropical hardwoods — bleed tannins that cause yellow/brown staining through water-based topcoats. Shellac BIN provides effective tannin blocking.
Soluble salts (efflorescence). On masonry and plaster, soluble salts from the substrate can crystallise on the surface and push through paint in white, powdery deposits. Standard primers do not block soluble salts effectively; specialist stabilising solutions (Sandtex Stabilising Solution, Dulux Trade Weathershield Stabilising Primer) are required.
Metal Primers
Metal surfaces — railings, cast iron radiators, steel window frames, metalwork balconies — require primers specifically designed for metal adhesion and rust prevention.
Zinc phosphate primers. The workhouse metal primer for most applications. Zinc phosphate reacts with the metal surface and any residual iron oxide to form a stable, rust-inhibiting layer. Apply after thorough mechanical preparation (wire brushing to remove all loose rust and scale).
Direct-to-metal (DTM) paints. For sound, previously painted metal in good condition, a DTM product that combines primer and finish in one product is a practical option. These do not replace proper primer on badly rusted or bare metal.
MDF: The End Grain Problem
MDF is ubiquitous in London fitted joinery — shelving, cabinet carcasses, architrave and skirting replacements — and it requires attention to one specific issue: end grain sealing.
The end grain of MDF is significantly more absorbent than the face. Without sealing, paint sinks into the end grain and produces a visibly different texture from the face — even after multiple topcoats. The correct approach is to apply a coat of diluted solvent-based primer or a purpose-made end grain sealer specifically to the cut edges, allow to dry, and then proceed with standard priming and topcoating of the whole surface.
When Skipping Primer Is Acceptable
Not every painting project requires a primer. On a surface in excellent condition — sound, well-adhered previous coats, no staining, no bare patches, no change in paint type — the finish coats alone will perform adequately. The question to ask is: what is the risk if I don't prime here? If the answer is low (good adhesion, no staining risk, no compatibility issue), the primer can be omitted. If the answer is high (bare substrate, stain risk, sheen incompatibility), skipping it is a false economy.
Good decorators make this judgement by surface, not by project. A hallway being refreshed over sound existing decoration may need no primer at all. A room being redecorated after water damage will need stain-blocking primer in the affected areas at minimum, and possibly throughout.