I'm not sure about Bondo, but I've used fiberglass and resin sandwiches made up
on Saran Wrap to patch a few times. Just make up the patch, stick it on and
mold it with your fingers. After it sets, the Saran peels off and leaves a
smooth surface. You may have to watch it for awhile until the resin "goes off"
to prevent running out.
Joe Savard
Lake Orion, MI