Is it OK to clearcoat the outside of your RDA cap
I have a few caps with AFC rings in both copper and brass and I have not coated any of them.
I find they do dull a bit after a while but they never get a real noticeable patina on them. I suspect it's because juice either protects the surfaces to some degree. Either way it works for me.
A lot of wood finishes are considered safe to eat from as long as what you're putting on it does not degrade the material. Wood bowls and eating utensils are
coated with a variety of things. Personally I would prefer varnish for vape parts. I mostly use Varathane on woodwork projects if I don't use oils, but I don't make bowls or utensils. My kitchen cutting boards get treated with a butcher block product that is food safe, but that wouldn't work for metal parts because it needs to penetrate. Ultimately I feel safest with nothing on these vape parts.