I agree that this may be coming from the resin itself, and recommend your first coat be a layer of etching primer to serve as a barrier. I use this to prevent bleed-through on kits molded in red plastic. Etching or "self-etching" primers are widely available these days in spray cans. I know my Harbor Freight, Wal-Mart, and Autozone have a version, and I'm sure you'll be able to find some. You may decant it and airbrush it, or some spray good enough right out of the can; you just need a thin even coat. Use your favorite light colored primer on top of this, and paint away.