You can apply a topside paint over slick, nonporous polypropylene, polyethylene, ABS, and PET/PBT surfaces by doing proper surface preparation and using an adhesion-promoting primer such as 3M Tape Primer 94 or Tesa adhesion promoter.
To prep a previous coating in good condition:
- Clean the surface: Remove surface contaminants by wiping with a clean, lint-free cotton rag dampened with denatured alcohol.
- Sand the surface: Sand the surface with coarse sandpaper (60-80 grit).
- Remove sanding residue: Solvent wipe the surface again with a clean, lint-free cotton rag dampened with denatured alcohol.
- WORKING ON A SMALL TEST SECTION: Apply the adhesion-promoting primer over a small section of the prepared surface, according to directions, and allow to dry completely. You may need to apply a second coat. If so, allow that to dry completely.
- Apply a small amount of the desired topside primer over the area coated with the adhesion-promoting primer. When the topside primer dries, do what's called a 'hatch' or 'crosshatch' test. Use a sharp blade to score a 2-3 inch line into the adhesion-promoting primer/topside primer. Repeat by cutting another three parallel lines the same length, about 1 cm apart. Then cut four perpendicular lines about 1 cm apart. It'll look something like this.
Apply duct tape to the scored area and yank the tape off quickly. If there's any topside primer or adhesion-promoting primer on the tape, the adhesion is poor. If there's no topside primer or adhesion-promoting primer on the tape, you can safely apply the adhesion-promoting primer (might need two coats), as directed in Step 4, above, and allow to dry completely. Apply your topside primer and topside paint, as directed.