Two things are an an absolute must when using acrylics:
1) CLEAN THE PARTS! Acrylics to not adhere to plastic like enamel or laquer, nor do they have the solents in them to dissolve any oil or grease on the parts. The parts MUST be free of oil or grease (including that from your hands) or the adhesion is going to be virtually nil.
2) PRIME THE PARTS. If you are going to mask over acrylic, prime the parts with a good quality enamel or laquer primer or you run the risk of the paint pulling up.