Originally Posted by steelbikeguy

I just went through this with a Superbe front derailleur that I've been using for 30 years.
I applied penetrating oil, applied a bit of tapping via a screwdriver (to try to break the screw loose), and repeated this a few times. No success.
I then tried soaking the derailleur in penetrating oil for days. This probably helped, but I still couldn't budge the screw.
Then I tried applying hot air to the derailleur body, with the hopes of causing the aluminum body to expand more than the steel screw. That did finally get the screw to move!

The hot air came from a "hot air rework station"... which is designed to generate air hot enough to melt electronics solder. It was hot enough to make the penetrating oil start to smoke!
I'm not sure what other tools can be used to generate this sort of heat, but it can be helpful in this sort of situation.

Steve in Peoria