Hi, I have always used the same trick as melv16 also having been a decorator but also put some oil based gloss in with it and it has always worked fine. As for painting over it with emulsion, just put the first coat on and allow to dry which does take a good number of hours then emulsion as normal. Although there are a number of sealers on the market these days available from most DIY stores. If you have some oil based paint why not try it and save a few quid.