Removing decals in Model Building Questions and Answers Posted November 7, 2023 Non-water based paints (the solvent-based "stinky" plastic-compatible paints like Testors) have solvents "hot" enough to "bite" into styrene without a primer, but not hot enough to cause crazing. When using those paints no primer is required (and I don't use it). All it does is to increase the total thickness of the paint, which is not a desired thing. But the plastic surface must be clean. I suspect that in the 13-year-old Bob's case, the plastic was not perfectly clean, which caused the paint not to bond to the plastic very well.