Stain block
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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
- Stain block paint
- Shellac
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 alternating coats of emulsion and stain block, if oil paint isn't used.
PVA should always be diluted when used. Otherwise it can form a slick surface that paint won't stick to.
The staining of walls by fire is a bit different to water staining, and aluminium primer is favoured. This forms a layer of aluminium flakes, blocking odour as well as soluble stains.