You will find some really awful paint materials in spray cans, and a few surprisingly good ones. It's hard to say which because they change their formulas from time to time. My last attempts with Rustoleum were dismal. My best work has been with real automotive paints (not in spray cans) and applied with a good spray gun and air compressor. And yes, disaster can strike at the last step - applying clear over the color coat, when the clear wrinkles the color coat. You are at risk due to materials incompatability, coat thickness, and the "recoat time" window. Some paints may be recoated in an hour or less, or else wait 30 days before recoating. Read the labels and try it on something else first.

As for using a hot car as a drying booth - The upholstery material can inprint its texture on surface of the semi-cured paint if the frame is left sitting on the hot seats. Might be better to hang the finished frame in the garage or in the house and leave it alone for a week or so.

This one was done with Candy Colors lacquer Clear, over Candy Colors blue color, over Candy Colors silver base, over lacquer primer. And nothing over the decals: