Re: OK, so I'm Sanding My Bottom...
I agree with the wide flexy plastic thingy. This is really an easy repair if you use epoxy, forget Bondo. Get some epoxy (you need about a qt) with those little metering pumps. Save a bunch of butter tubs for mixing it in. Get the reddish plastic low density microballoon filler. Get some of those cheap chip brushes from Wal Mart, get a bunch of em. Mix some epoxy unthickened and paint it in the cavities to seal them. Next, mix up some epoxy thickened with the filler and put it in the cavities using the plastic thingy to spread it to shape of the hull.
Something I'd try, tape wax paper over the craters on top of the epoxy. The wax paper does not stick to the epoxy and will give you a smooth surface.