Nice job. Given the age of the bike, you may well have a real enamelled frame. That could explain why the paint didn't bubble.

Real enamel is a glass-like substance, baked on the frame in a similar manner to today's powder coating...but MUCH more durable. Modern "enamel paint" was developed as a cheaper, but less durable substitute to real enamelling, and will almost always bubble and burn when you torch it. If your bike has a real enamel finish, it's another reason to keep it and restore it.

Good luck with the fixed cup.