You need to be using good quality plastic primer and a real good keying. The cold is fine, its the damp wont be helping, with the plastic primer you just have to "SHOW IT" the paint, very very thin, then you can use normal primer over the top afterwards.
If your using that halfords stuff, its silly thin.......uber uber light coat. If in doubt, send it my way and ill do it for £15 returned (pm me) any colour you want, same offer for anyone else who wants diamonds doing. (Recently done many bits on my mates car including diamond, the paint wont chip or flake that i guarantee, im not using cans or halfords paint, premium quality automotive).