How do I paint over a stain that keeps coming through?

Adding coats to a stain that keeps reappearing is the classic decorating trap. More paint has never once fixed it, because the mechanism is chemical rather than a matter of opacity.

Safety

Step by step

1. Find the cause before anything else.A water stain means water got there. Roof, gutter, pipe, condensation, an overflow. Painting over an active source hides it while the plaster carries on deteriorating.
2. Make sure it is fully dry.Weeks, not days, after the leak is fixed. A moisture meter compared against an unaffected wall is the reliable check. Blocking a damp stain traps the moisture behind the coating.
3. Understand why emulsion fails.Nicotine, water-soluble tannins, soot, some dyes and marker ink dissolve in the water in emulsion and are carried up into each new coat. It is migration, not poor coverage.
4. Identify what you are dealing with.Water and damp stains are brown and diffuse. Nicotine is a yellow-brown film over whole walls. Tannin from timber is sharp-edged around knots. Soot is grey-black and greasy. Each needs the same class of product but different preparation.
5. Clean it first where relevant.Nicotine and soot should be washed with sugar soap and rinsed before blocking. Blocking over a greasy film gives the blocker nothing to grip.
6. Choose the blocker.Shellac-based is the most reliable for nicotine, tannin and stubborn water stains, and it dries very fast. Solvent-based blockers work well and smell strongly. Water-based blockers are convenient and less effective on the worst cases.
7. Do not use ordinary primer.Primer improves adhesion; it does not stop migration. This is the most common mistake and it wastes a whole extra coat.
8. Cover the whole affected area plus a margin.Well beyond the visible edge, because the stain has spread further than it shows. Spot-blocking exactly the visible mark leaves a ring that reappears.
9. Apply two coats if the stain is strong.Let the first dry — shellac is often ready in under an hour — and check whether anything is showing through before the second.
10. Block the whole wall for nicotine.Nicotine is rarely confined to a patch. Blocking one area and painting over the rest gives a visibly different finish and the surrounding stain still bleeds.
11. Check before topcoating.Look at the blocked area in good light after it has dried. If any shadow of the stain is visible, add another coat now rather than discovering it through two coats of colour.
12. Then paint normally.Once properly blocked, ordinary emulsion goes over it with no special treatment. Two coats for even colour.

Tips

• Ordinary primer does not block stains. That single misunderstanding accounts for most repeat failures.
• Go well past the visible edge. The stain has travelled further than it shows.
• Check the blocked patch in good light before topcoating. Fixing it then is one coat; fixing it later is three.

Common mistakes

• Adding more coats of emulsion — Soluble stains dissolve in the water in emulsion and migrate into each new coat. Coverage is not the problem, so more coverage does not help.
• Painting over a stain that is still damp — It traps the moisture, the coating fails, and the underlying plaster continues to deteriorate out of sight.

If it doesn't work