hi, I am going to paint new plasterboard, (not skimmed) and I have a huge quantity of Dulux Rich matt white emulsion. It sells it'sself as having a 'wipe clean' finish but it is still matt. Anyone know if I can thin this with water to use as undercoat? top coat will probably be silk/eggshell finish.
Thanks!

I wouldn't risk it as it will change the consistancy and so the durabilty of the paint if you water it down.

Just raw plasterboard, the paper finish?  This must be sealed properly, usually with a thin coat of plaster.  If you're not doing that then take advice from the manufacturer about the best product or use a dry wall sealant such as recommended in this article.

Once you've sealed the plasterboard then I can't seen any reason why you shouldn't use your emulsion as an undercoat, or just as a first coat.  If it's a water based paint then yes, thin with water.  Check first though - if you've ever added water to an oil based paint you'll know why!!  You may find instructions on the tin about thinning for a first coat, or look it up on line at Dulux.  Also make sure that your undercoat and top coats are compatible ie both water based or both oil based.  Have fun :)

You can dilute emulsion 25% to seal new plaster (as long as it is really dry) but I'm not sure about just plasterboard ???? We have always had plasterboard skimmed.

Daisy

Would have thought a coat of PVA would seal the plaster board, you could use thinned paint but it may soak into the board and cause a problem.

PVA would still need thinning 5:1, but under paint, dilute emulsion is better. We use dilute PVA before tiling floors/walls etc

Daisy

thanks all for your ideas
Daisy: Do you know if rich matt (wipeable) emulsion is any different re thinning to bog standard emulsion?? it is definitely water based as it say to thin to 10% in hot weather to ease application.