"I see a lot of reference to MDF as a mount but nobody has said how deep it should be. Seeing as you can get it in anything up to 1" or more (25mm +) it really is relevant. I have some stuff here that is 1/4" (6mm) and would make a large painting base. Thin MDF may be susceptible to damp. Any ideas?" -- Jim
I don't think the thickness of the MDF matters much except that thinner boards may require bracing on the back to stop them warping and thicker ones will get very heavy in large sizes! Prime both sides (not just the one you intend painting on) as well as the edges to seal the board. This will help protect it against damp getting into the board. Continuing your painting around the edge will add another layer of "sealant", as could varnish. See Painting on Hardboard or Wood for more tips on how to paint on a board rather than canvas.