How fast should the buffer be running? Slow! Around 600-900 rpm's. But why? The buffing pad and compound should be abrasive enough to actually cut the paint, without the use of heat, or very little heat. The best method is to use a good and aggressive compound and a good quality wool pad. Heat isn't required if the paint has been sanded thoroughly to a fine enough grit.
Why is heat bad? There are several reasons, especially when it comes to vehicles that have been restored, or hand built and body worked.
- The first is that heat in a concentrated area could cause the metal to move around, which could actually put a wave in the panel.
- Heat could cause any issues in the undercoats (filler or primer) to make themselves present. If there's an air pocket in the filler, or a scratch hiding in the filler, or any number of other things, heat from buffing to fast could easily make these issues pop up. And yes, in an ideal world this shouldn't happen, but the reality is that it is all too possible.
- The paint. This is mostly just true with new paint, but forcing too much heat into the new paint could very easily force too much solvent out of it at once, and cause it to die back, or even solvent pop.
Just buff slow! You will get much better results, and just as fast!