What is primer?

You’ll always, ALWAYS, want to prime your sculpts before painting them. (Assuming it’s not translucent.) Currently I use white primer by Testors, available at hobby stores. It dries reasonably well on most plastics. If it remains a bit tacky, I’ll follow up with a coat or two of dullcote. (And don’t forget to spray in a well ventilated area, and/or with a respirator of some kind.)