My wife is a chemist who has worked professionally with polyesters.
She reports that a good product results only from proper cross linking
in the polyester molecules.

There are several factors that could result in bad cross linking,
including contamination, bad mixing, and incorrect ambient temperature,
but the key problem seems to be the use of old (deteriorated) benzoyl
peroxide catalyst.

If you use old Bondo that has sat on the shelf at the auto parts store
or in your shop for years or even months, expect that the resulting
polyester will disintegrate.

Peter Neilson