Water stains aren't a colour problem, they're a chemistry problem. Ordinary emulsion is water-based, so the water-soluble salts and tannins in a stain simply dissolve into the new coat and travel back to the surface. More coats of emulsion just feed it.
First: deal with the cause, not the mark
The stain is a symptom. Before any decorating, the source of the water has to be found and fixed — a leak, failed flashing, an overflow, condensation — and the area has to be allowed to dry out fully. Painting over an actively damp wall is wasted work; the moisture will keep pushing back through whatever you put on top. This is the step that's tempting to rush and the one that matters most.
Then: prepare the surface
Once the area is genuinely dry, it's prepared like any sound repair:
- Remove any loose or blown plaster, and skim-finish where the surface needs making good.
- Rake out cracks and take off loose or flaking paint.
- Fill cracks and blemishes with the appropriate filler and sand back flush.
The key step: seal the stain
This is where the job is won or lost. A water-based emulsion will not hold a water-soluble stain back — you need a barrier the stain can't dissolve into. On damp-related and water-stained areas we apply an alkali-resistant primer and an appropriate oil-based stain killer, which lock the stain in and stop it bleeding through. Only then do two to three coats of finish paint go on. Sealed properly, the mark stays gone.
The same principle applies after smoke and fire damage, where a heavy-duty stain-blocking primer-sealer such as Zinsser B-I-N is used to shut down stubborn smoke staining and odour before redecoration. It's the reason a sealed job stays clean and an unsealed one doesn't.
The short version
Fix the source, let it dry, then seal before you paint. Skip any of those three and the stain comes back — which is exactly why "just paint over it" so often fails.