I always clean the Vinyl or Plastic with Acetone or some other high strength solvent that will not leave a residue.

It will "soften" the surface and helps the paint/dye to sink in better. I died a set of White seats in my Challenger black. Even as a daily driver with lots of use the dye stuck pretty well. Only when scratched with something hard and sharp, did the white show through. Normal wear and tear had no problems and they stayed black even on the seat bottoms for years.

Clean it right before you paint them so the surface is still soft when the paint is applied.

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