bumps in fiberglass

I use 60 or 80 grit for this step. Just make suer that you don't wait too long for it to dry. If it starts creating dust when you sand, it's too late and you will need to sand for quite some time. The texture of the body filler should still be some what rubbery but not to the point where your first pass drags it right back to the last layer. The sand paper will fill instantly with body filler, but that's OK. Good luck and post your results.

I usually take about an hour off sanding for me when I do it this way.

I did this once as well, the problem is on those curves or rises that you want to keep. I hardly use my DA sander any more since I learned this technique about 2 years ago. It's straight as an arrow and takes time off the project by at least 50 % ! Give it a shot the next time you use body filler.