The purpose of sanding is to give the tubes a good finish and surface prep them for painting. IT's important to start by putting a coat of epoxy on the tubes - this helps fill in any holes, and it makes sure that the fiberglass itself isn't sanded, weakening the structure. We would start with 120 grit sandpaper to remove the large imperfections. We increased sandpaper grit up to 400 or 500 grit, to make it as smooth as possible. Remember to wear a respirator when sanding composite tubes - epoxy and fiberglass or carbon fiber dust are dangerous to inhale
Surface prep depends on the epoxy system used. For West Systems, the epoxy can be sanded and will adhere to Bondo.