I could use STP as hair pomade. It would keep my hair in place. I don't. There are accepted products formulated to overcoat, protect, and restore the appearance of automotive paint just as there are similar preparations for fiberglass gelcoat. When the conventional solution fails to produce a reasonable benefit ratio against cost of product and time/effort, it's time to look for unconventional solutions; not before.
I'm not a chemist and I don't play one on this forum. There are questions of compatibility (Will it stick?) and reversibility (Is it difficult to remove if necessary?) which apply to any overcoat on any surface or over another "finish." There is at least one frequent contributor here who has applied RMPro to automotive finishes and to thermoformed plastic fenders, rocker panels, etc. and is happy with the results. About a month back, another member here and fellow Burro
owner stated that, in his experience, attempting to remove RMP or similar from a painted finish with ammonia created a streaked mess on the lower portions of a painted trailer.
ALL endorsements AND condemnations of unconventional solutions for familiar problems need to be taken with a block of salt. In Jude's case I believe it prudent to "try in an inconspicuous area" or, better still, on a disposable item such as an orphaned or junked fender, tailgate.