If you have ever had a leak come through a painted ceiling or an old wall has been damp in the past, you will know that it does not matter how many coats of emulsion, or how many layers of wallpaper you put on, the stain will still come through. The only way to stop this happening is to cover the stain with a stain blocker or an oil based paint first. You can then emulsion or paper.
You can make a stain blocker by mixing some oil based undercoat and gloss together and painting this over the stain first. Proprietary stain blockers can be bought by clicking through to our tool store and searching.
A ceiling may have leaked in the past, the leak been fixed and the ceiling may even have been plastered over......Those old water stains will keep coming through until and oil based coat is applied first.