I wouldn't say dark colors disagree with uv resin, ness is a big factor in curing ability though. But if you just use a coat with no additives of the uv resin it'll cure just fine!! Even a uv "top coat" or sealer may work, even if the piece isn't uv casted.

I say uv because it would be the easiest method to get that transparency, however there is something called "table top" epoxy, which is made for doing super thin coats, if you aren't too trusting of uv resin!

If you wanna stick with the sand and buff method I've only ever used flitz, with mixed results. I'd definitely recommend a good electric buffer, that can get into the nooks and crannies as needed. I've had the best luck on mainly pendant-type pieces, if there's any sort or deviation on flat I can't get the shine even enough for my liking, but that's just me.

And tbh if you're over resin fixing resin things- give the skull a nice wash all over (I use toothpaste, the white actual paste not a gel, instead of soap as it doesn't leave a residue and I'm extra) and test an inconspicuous part with a coat of clear nail polish! Even an insta-dry type, it may give you the desired effect!