The heat gun method works - for a while. The better dressings work - for a while. Paint also works - until its time for another coat of paint.
Pick your poison. Some methods are quick, some take hours. Some are cheap, others aren't. None reverse the oxidation process.
I generally try to avoid painting fender flairs until nothing else works because once you paint 'em black you can't go back. But that's just personal preference. I have also avoided the heat gun method because of the many reports that over time it dries out the vinyl which can in turn actually accelerate oxidation.
I've used Meguire's Ultimate Black Plastic Restorer with acceptable results, but for fender flares and vinyl trim still in reasonably good shape I've found that all that is really required is a quick wipe down with a sponge or cloth dampened with my preferred tire dressing: original satin finish No Touch Tire Care. Cheap, easy, and takes only a few seconds.
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