Damp Seal Calculator — Stain Block Primer Quantities

Stain block and damp seal primers are formulated to be more opaque than standard paint, which reduces their coverage rate. They contain shellac, alkyd resin, or specialist polymers that create a barrier against stains bleeding through. Manufacturer coverage figures range from about 9.8 m²/L (Zinsser Cover Stain) up to 12.5–14 m²/L (Zinsser B-I-N, Dulux Trade Stain Block), against up to around 17 m²/L for a standard trade vinyl matt emulsion. Always check the data sheet for the exact product, as porosity and surface texture reduce coverage in practice.

Two coats are recommended for effective damp sealing. One coat may be sufficient for light water stains, but two coats ensure a complete barrier against persistent stains, nicotine, or tannin bleed. For severe damp staining, apply two coats of a shellac-based product like Zinsser BIN. Allow the first coat to dry fully before applying the second.

It depends on the stain. For nicotine, smoke and stubborn water stains, Zinsser B-I-N (shellac-based, 12.5 m²/L, recoat in 45 minutes) is the trade standard. Zinsser Cover Stain (oil-based, 9.8 m²/L) handles tannin and water marks and works inside and out. For a fast, low-odour water-based option, Zinsser Bulls Eye 1-2-3 (10 m²/L) primes and seals most surfaces. Dulux Trade Stain Block Primer (up to 14 m²/L) is a good plaster-wall stain blocker. Dulux also sell a retail product literally called Dulux Damp Seal Primer & Undercoat (a matt water-based primer/undercoat, up to 8 m²/L, touch-dry about 4 hours) for sealing dried-out damp patches before topcoating. For genuine damp or masonry waterproofing, Zinsser WaterTite is a waterproofing coating rather than a stain primer — note coverage is much lower than a normal primer (about 2 m² per litre per coat, per the Zinsser UK WaterTite product data sheet), so two coats on porous masonry use roughly four times as much product as a standard primer. Zinsser do not make a product simply named 'Damp Seal' (their damp range is Damp Proof Paint and WaterTite).

You should never paint directly over actively damp walls — the paint will peel and mould will grow beneath it. First fix the source of damp (rising damp, penetrating damp, or condensation). Once the wall is dry or has been treated, apply a damp seal primer before painting. Damp seal is not a cure for damp — it's a barrier to prevent stains bleeding through once the underlying issue is resolved.

Most stain block primers give a minimum recoat time rather than a maximum window. Zinsser B-I-N is recoatable from 45 minutes, Bulls Eye 1-2-3 from about 1 hour, Cover Stain from 2 hours, and Dulux Trade Stain Block from 6–8 hours. These shellac and oil primers do not publish a maximum overcoat deadline, so you can usually topcoat any time after they are fully dry. If a previously primed surface has been left a long time or contaminated, lightly key it before recoating. Always check the product data sheet for the exact recoat interval.