When I got my truck, it had some exposed metal on the rear of the cab. I sanded all the rust off it, chucked some primer on it, waited a bit and then chucked some bright blue over that. 10 months later, I sanded back the paint, repaired the metal, filled etc it and then primered it all. I left it to dry and came back to something that looked like this:
It took me half a day of sanding, primering, sanding, primering etc to get rid of all the reactants in the paints as all the sanding was bringing up more of the reactants
Always use products which are designed to go together, otherwise you'll be cursing for a long time