What is stain block, and when do you actually need it?
If a stain has come back through three coats of paint, a fourth won't fix it. You're not covering the stain, you're re-wetting it. Here's what's really happening, and the one product that stops it.
Why a stain comes back through paint
Emulsion is roughly half water. When you roll it over a water stain, nicotine, or a timber knot, that water does exactly what water does: it dissolves what's beneath it.
The dissolved stain then travels upward through the wet paint film by capillary action, and as the coat dries it deposits itself at the surface. You've moved the stain, not covered it. Do it again and you move it again. This is why the fifth coat looks identical to the first, and why people conclude the paint is rubbish when the paint is behaving exactly as designed.
Stain block breaks the cycle by putting a physical, impermeable barrier between the stain and everything above it. The stain is still there. It simply can't get out.
Which stains need blocking?
| Stain | Needs blocking? | Recommended type |
|---|---|---|
| Water / leak stains (ceilings especially) | Always | Oil or shellac |
| Nicotine / smoke damage | Always | Oil or shellac, whole surface |
| Timber knots and resin bleed | Always | Shellac knotting |
| Mould staining (after treatment) | Always | Oil or shellac |
| Felt-tip, biro, crayon | Always | Shellac |
| Soot / fire damage | Always | Shellac |
| Rust marks from fixings | Yes | Oil or shellac |
| Grease / cooking splashes | Wash first, then usually yes | Water-based often enough |
| Scuffs, dirt, general marks | No | Sugar soap and paint |
| Old dark colour you're covering | No | Just more coats, or tinted primer |
That last row matters. Covering a dark red wall with white isn't a stain problem, it's an opacity problem. Stain block won't help much and you'd be buying the wrong product. What you need there is more coats or a tinted undercoat, which we cover in how many coats of paint you need.
Water-based, oil-based or shellac?
Water-based stain block
Low odour, easy cleanup, dries fast. Genuinely useful for light marks and mild discolouration. But it's water-based, which means it has the same fundamental weakness as emulsion on a serious stain: it can reactivate what's underneath. For a proper water stain, it will often let you down.
Oil-based stain block
The reliable workhorse. Strong barrier, handles most serious stains including water damage and nicotine. Smells, needs white spirit for cleanup, dries more slowly. For most household stain problems this is the sensible choice.
Shellac-based stain block
The strongest option and the fastest drying, often recoatable in under an hour. Shellac is the traditional answer for timber knots and it's what we reach for on the genuinely stubborn things: heavy nicotine, fire damage, permanent marker. It has a strong smell and needs methylated spirit for cleanup. It's also brittle, so it isn't for flexing surfaces.
How to apply it properly
- Deal with the cause first. Fix the leak. Treat the mould. Sealing over an active problem guarantees a callback.
- Let it dry completely. Sealing damp in is worse than the stain.
- Clean the surface. Grease under stain block stops the stain block bonding, and now you have two problems.
- Apply generously, and go past the stain. Cover an area larger than the visible mark. Stains spread further than they show.
- Two coats on anything serious. One is a gamble on heavy water or nicotine staining.
- Then paint over it normally. Stain block isn't a finish. It's a barrier that lives under one.
When you don't need it
Plenty of decorators will add stain block to every quote because it's an easy line item. You don't need it for:
- Ordinary scuffs, fingerprints and general grubbiness. Sugar soap.
- Covering a strong colour with a lighter one. That's coats, not sealing.
- Fresh plaster with no staining. That needs a mist coat, which is a different thing entirely.
- Filler patches. They need spot-priming for porosity, not stain blocking.
If someone quotes stain block across a whole clean house, ask them which stain they're blocking.
Why it's a separate line on your quote
Stain block appears as its own itemised line in our instant quote, with an explanation of what it is and why it's there. Two reasons for that.
First, honesty. If you're paying extra to seal a water stain in the spare-room ceiling, you should be able to see that decision, priced, rather than find it absorbed into a round number. You can look at it and decide.
Second, it's genuinely optional in a way most prep isn't. We'd never let you skip filling or sanding, because the finish depends on it. But if you know that ceiling stain came from a leak you fixed years ago and you're relaxed about it, that's a legitimate conversation. What we won't do is paint over it, say nothing, and let you discover in six weeks that it's back.
Got a stain that keeps coming back?
See a fixed price with any sealing itemised and explained, so you know exactly what you're paying for.
See My Price Instantly →Frequently asked questions
What is stain block?
Stain block is a sealing primer that forms a barrier over a stain so it can't travel up into the paint above. Ordinary emulsion is water-based and porous, so water-soluble stains simply dissolve and rise through each new wet coat. Stain block puts an impermeable layer between the stain and the finish.
Why does a stain keep coming through paint?
Because you're re-wetting it. Every coat of emulsion is mostly water. That water dissolves the stain underneath, the stain travels up through the wet paint by capillary action, and reappears as the coat dries. This is why the fifth coat looks exactly like the first. The only solution is to seal the stain rather than cover it.
How many coats of paint to cover a water stain?
None will work. Water stains contain dissolved tannins, salts and dirt that are water-soluble, so more coats of a water-based paint simply reactivate them. One coat of stain block will do what ten coats of emulsion cannot.
Do I need oil-based or water-based stain block?
Water-based is fine for light stains such as minor scuffs, small marks and mild discolouration, and it has far less odour. Oil-based or shellac-based is the reliable choice for the difficult ones: water and smoke damage, nicotine, timber knots, felt-tip and biro, and mould staining. Shellac is the strongest and dries fastest, but needs methylated spirit to clean up.
Can you use stain block over damp?
No. Stain block seals a stain; it doesn't stop water. Applied over an active leak or damp wall it will fail, often by blistering, because the moisture pushes it off from behind. The source of water must be fixed and the wall fully dried before any sealing or painting.