Received this from 'OldHarry' which I found quite useful:
Most of the better masonry paints (i.e not cheapo Homebase type which is too soft and wears quickly) seem to stick to pretty well anything. The criteria on the tin is that the surface is firm and dry, so a good wirebrushing will probably do most of it. I have always used it over other paints. Its water
resin so doesnt seem to react with any oil
based paint. I would have thought it would stick to GRP - I had no end of trouble getting it off when I got some direct on Gel coat where it wasnt needed and let it dry. However I would have thought a good rub down with say 120g to take any surface gloss, and remove any wax/silicone polishes that may have been used would be a wise precaution, as well as keying it sufficiently for the paint to stay put. That old red treadmaster looks 'orrid when it has weathered badly, doesnt it? It too should take the Sandtex or whatever if its still serviceable other than the fading. Its not fun to get it off either, as the glue used to fasten it doesnt respond to anything short of real brutality! Hope that helps.