yes, it does work. we've had to do this twice now, due to a leak from a shower above the hall. a light brushing of oil based undercoat before applying emulsion paint really does stop the stain from reappearing.
Hi twinny, don't use just the undercoat as the chances of that working are slim. Mix oil based undercoat and gloss together 50/50 and paint this on. Paint it to an area a bit bigger than the stain allow to dry, emulsion over the patch and then give a couple of coats of emulsion once that has dried.
would have to disagree with you shedman :)
oil based undercoat has always worked for us, as recommended by a decorator we employ from time to time.
we had a boiler moved in the kitchen and a watermark was left by the balanced flue, twice we've had a leak from a shower on to the hall ceiling, and a large watermark was left on the ceiling and part way down a wall in the utility room by a broken overflow pipe. on all these occasions the watermark has been completely eradicated by using oil based undercoat before emulsion.
ethandron the gloss will stop the stains coming through better than undercoat but you need the undercoat mixed with it so that it helps the emulsion to stick otherwise the emulsion would just sit on top of the gloss and not dry properly. I worked as a decorator for many years and was taught this by one of the best and it has never failed. As with all things though different people have different ways of doing things. This was just my answer to the question asked. Forgot to put in my first answer to make sure as chokkie said that the problem had been dealt with before painting.
dear mr. shedman, i can only speak from experience and assure you that oil based undercoat, without the addition of gloss, works a treat and has never failed us.
i'm not disputing that the addition of gloss also works....but in my experience is not necessary.
personally, i assumed that twinny had sorted out whatever caused the watermark, as most sensible people would.
anyhow, each to their own :)