Hi
Hopefully you can see two images, the pink one is the new Sandtex terracotta, the redder one is what I'm painting over. You can see that the Sandtex isn't sticking to the older red gloss, so what to do.
There's a lot of it, so if we can get away without "just remove it" that would be good.
Background: the house is painted with Sandtex from years ago but it's starting to peel off. The rendering is fairly sound so we've opted for painting.
At first floor level (life may be easier as I get higher) there's rendering, then a sort of maroon gloss, then a bitumen/tar layer, then a white, then the old pink Sandtex.
I've scraped away at it, much of the top Sandtex is just falling off, the bitumen layer comes off ok .. I thought that would be a problem but paint seems to stick to it, but then there's this maroon gloss which, if you look at the mainly pink photograph, you can see this is the day after being painted and left overnight, it's just not stuck to the gloss at all.
I'm hoping you'll say .. ah, Sandtex primer is what you need, but just bear in mind the substrate it has to cope with is this problem mix of render, old probably Victorian probably lead gloss, and the bitumen tar.
Advice required :-)