I am about to get paint and rust off of an old steel Peugeot frame, and I have been recommended "Naval Jelly". I have never heard of it, but found it at my local hardware store. I have been told it's particularly good at getting rust/paint combo off.

I have also been told that wire brushes strip a patina off and weaken the frame - i have heard this before. One more time and it qualifies as "true".