I have some pics of my fiberglass bumper repair posted here:
http://www.251.org/rust.html  The damage was limited to a crack in the
center of the bumper blade.  The crack was filled on the top and then sanded
through from the back to the new material, and filled again from the back.
The bumper turned out very well, the repair is not visible.

Dave
251.org
87 Vanagon
61 Microbus