Specifically, I've drawn a parabolic arc which I've turned into a flange (it's a reflector). Now I want to add mounting holes. I can't sketch on a curve; and when I "unfold" the flat pattern after adding holes, the holes disappear.
Admittedly, sheet metal is not my strong point - I do more with machining.
Use the "unfold' command, add the holes or cuts, then use the "fold" command.