A water stain is a tannin and salt problem, not a coverage problem. That brown or yellow halo is water-soluble residue, so the moment you brush a water-based emulsion over it, you reactivate it and it bleeds straight back through — sometimes worse than before. You can put six coats of Dulux on it and it'll still ghost through. The fix is to lock it down with a solvent-based stain blocker so the topcoat never touches it.
Reach for Zinsser BIN. It's a shellac-based primer-sealer and it's the gold standard for water stains, nicotine, smoke damage and timber knots. It dries in about 45 minutes, blocks almost anything, and you can paint over it with any finish. One coat sorts most ceiling stains; give a stubborn or very dark mark a second coat. Zinsser Cover Stain (oil-based) is the alternative if you want something with a bit more body and don't mind the longer dry time — but for sheer blocking power, BIN wins.
Two non-negotiables before you prime:
Find the leak first. A stain means water got in. If the source is still active — a slipped tile, a weeping pipe, a failed seal around the bath — no amount of BIN will save you. Sort the cause, let everything dry out properly (a damp meter helps), then block.
Ventilate and mask up. BIN is high-VOC and the fumes are strong. Open windows, and use a proper mask, not a paper one. Clean your brush in methylated spirit, not water.
Once the stain's sealed and dry, paint as normal. On a ceiling I'd go with a flat matt — Little Greene Loft White or a soft white like Farrow & Ball Wevet both sit nicely overhead without glare. If it's a wall, match your existing scheme.
Don't skip the blocker to save a step. It's the one job where a £15 tin of BIN saves you repainting the whole wall a week later.