Yep, Bondo Spot and Glazing Putty is just the ticket for cowl seams. Dries fast, sands very easy and gives the best feathered edges there are. That's what it's made for.

But as for microballoons, I use the stuff all the time mixed with 5 minute epoxy. How easily it sands depends on your epoxy/microballoon ratio. Mix the epoxy first, then add microballons until the mixture is a nice fluffy white and it's as thick as toothpaste or thicker. It'll sand like a dream. You need to work fast though or use slower epoxy.

Dave