Resin/paint problem

Hi all hope you can help, iv come across a problem that I need help wit, on a couple of occasions when painting cas resin, the paint doesn't dry and becomes sticky, even after sanding back and reprinting it some sort back, iv had one also where the resin has gone soft and iv had to dig it out and rebuild, thinking I'd not mixed he resin properly, however Iv noticed it on a a couple of slush casts also, I would have thought even if I hadn't mixed it enough before pouring when slushing it round it would continue to mix it, the paint I use are general halfords spray paints, I have even tried plastic primer instead of the standard stuff, Any ideas ?Ta in advance

Sounds like bad resin if it's still doing it after properly mixing.
Also did you wash and primer the parts before painting?

I use lacquer automotive primer.

Has happened with a couple of different batches though, although they are from the same company, always thoroughly was the cast resin and sand to get rid of mold release

If it's from the same bottles of resin for all your castings than the resin is bad.

Wash parts in warm water with a good degreaser like dawn, soak and scrub.
Rince and let dry.

I have the same problem on one of my casts, but I learned that it was most likely because I didn't let the mold degas long enough - I poured the first cast only two days after moldmaking.

Another possibility is moisture - PU appearently hates water, even exposure to very humid air before beiing fully cured can hinder the chemical process...

Think about if there's anything in your workflow, maybe you cleaned the mold with water before casting and it was not fully dry, or maybe you cast in a very humid basement, or anything like this?