Stains on emulsion paintwork often bleed through when painted over, and repeated coats of emulsion fail to block them. A coat of any of the following can stop this happening:
- Dilute PVA
- Oil based (alkyd) non-gloss paint, such as undercoat or eggshell
- Acrylic paint
- Stain block paint
One or 2 coats is normally enough, but not always. Water based stain blocks aren't as powerful as oil based paints, and the very worst stains can require either oil based paint or alternating coats of emulsion and acrylic.