lus1:are you filling a low or repairing a hole?
for filling a low, roughen with 80 grit sandpaper.. if its a shallow low (less then 1/8 of an inch i would use bondo or JB weld as a filler. apply and get a **slightly** oversize profile. sand down and your good to go. if you use JB weld (which is better in some cases) you have to be careful with your sanding as it is harder then the plastic and will leave a low spot around your orgonal low.
for deeper low spots use the same roughening procedure but insted of bondo, use Fiberglass and 2 part epoxy (loctite hysol 120hp is my personal favorite) and 3 oz hobby grade fiberglass. basically just layer it up, making sure to work the epoxy into the glass. build ot a slightly undersize profile then finish it off with bondo and a wetsand
filling a hole is basically the same ans a deep low except you rouughed the back side yo about 3/4 of an inch beyond the holes edge and cut in a little step for the epoxy to grab to in the front. use tape as a backing in the front ( Tyvek tape or other olefin type tapes work VERY well for this as they wont allow the epoxy to bleed and you can tension it)work the epoxy into the front step then just fill like you would for a deep low. finish off with epoxy and a wet sand