Most likely, there is still some resin "residue" present. Or like Steve mentions, dust from the resin casting process. WITH any resin cast part, wash the heck out of it with soap and water BEFORE painting. This is a really important step. The resin "residue" will prevent the paint from sticking or even curing, and could cause further problems like the dreaded "orange peel" effect.
http://en.wikipedia.org/wiki/Orange_peel_%28effect%29