Basically from what I've noticed, it boils down to this. If I rolled it, I can still dent it with my fingernail but it does not come all the way off.

On the stuff I sprayed with a HVLP gun, it is hard, and glossy, but if I push hard enough I can get it off with my nail, but it comes off so easy once I break the surface, that I think it's an adhesion issue. I did not spend much time prepping my door jambs. I probably should have hit them with 220 to give the paint some teeth, but I didn't. The paint I sprayed seems hard enough (or would probably be very hard given 4+ weeks of curing) but it was my fault I didn't prep the surface well enough for good adhesion.

Under my hood, I did hit with sandpaper (180 maybe) before using the rechargeable spray gun, and that paint is very hard and adhesion seems good.

Alas it's not scientific since many variables changed between application methods (roll, rechargeable spray can, HVLP + compressor), but I would venture a guess that good surface prep + sprayed rustoleum would turn out well and be hard and glossy enough for an old car.