Countersinking the holes on the outer skins is probably the most terrifying part of working on the skins. If the countersunk hole is too large, the screw head will blast right through, ruining the outer skin. Tonight I worked up the nerve to start countersinking the front outer skin.
I went slowly and iteratively, trial-fitting the screw after a few seconds of drilling.
Once the hole was just large enough, I screwed down the screw.
It took a while since I went so slowly and carefully, but the front skins are pretty much done, with no casualties. I may go back and drill a tiny bit more material out on a couple of the holes where the screw heads aren't quite flush with the surface of the front outer skin, but I'll work on the back outer skins before I do that.