I suggest that all unpainted surfaces be lightly primed with a Krylon or Rust-oleum primer gray primer. In my opinion the white primers have too much pigment so I don't recommend using them. If you need a fine primer (so as not to have the pigment fill detail linework), get an automotive gray. We are painting laser cut acrylic plastic. All flat surfaces are lightly sanded to help the primer grip. I find that, to avoid disappointment, laser cut window openings that are going to have sills masked for a cast stone color also have to be lightly sanded as the laser cutting leaves a polished surface and it is very likely that, even with primer, the paint may peal when the masking is removed. We use 120 grit sandpaper mounted on small sanding sticks. I do not typically work with painting/masking/painting cast resin parts, but I agree with the suggestion above that they have to be thoroughly cleaned and dried first. Ours get dunked in lacquer thinner, dried and then thoroughly washed in a Dawn detergent bath, rinsed and left to dry for several days.
As for Krylon and Rost-olem "2 in 1" colors, don't believe it. Any product that is labeled "PAINT + PRIMER" does not understand the concept that priming actually comes first.
As for masking, Nichiban tape is our masking tape of choice. For true clean work, over mask and cut your line line with a sharp Excel or Exacto hobby blade. All tapes are slitted from larger rolls and the edges are never as clean as your hand with a fresh hobby blade.
And as I always say, practice technique and paints on something that does not count first.
