I bought a couple of those hard rubber-like sanding blocks. The one's I got are called Dura-Blocks..one 6" and one 12"..They had them at the automotive paint store...they also have the stick-on sanding strips. I think i started with 60 or 80 grit to work the filler down and eventually got to 400 or 600 for the final sanding.

A Random Orbital sander is a big help also.

If you are removing old paint and bondo wear a respirator and use a propane or MAP torch and a putty knife...heat will remove bondo and paint...

Keep an extinguisher nearby....

It is messy and smelly and smokey... be sure to get a good respirator...

Also there are rust and paint removal wheels for the 4 1/2" side grinders that are pretty effective, but they don't last to long...

Wear ear protection....along with the respirator!

If there is anyway you can swing it, I'd take it to a media blaster....I wish I had!