The Vallejo primers take a little longer to fully cure than the rest of Vallejo's paints, usually 48hrs or so to fully cure (their regular paint is only about 24hrs). You can paint over the primer well before that, but if you need to sand anything, let if fully cure first, and it won't flake up.

Proper plastic prep will also help with that, IE: wash your parts to get the mold release off of it.