chalking surface.

I'm new on the forum so I apologize if this is covered somewhere but I couldn't find a thread on it. I have made some parts for a car club near me. I'm using tinted water clear resin and pressure casting. I hand polish the parts before sending them out but I've been told that after 1-2years the parts are becoming chalking on the surface. Should I clear coat them or should I be using a different resin.