Bondo is just a brand (the original brand I believe) of polyester body filler. 

I keep hearing references to this "Bondo" -- What is that exactly?   Some kind of Polyester filler/primer?

I use cheap polyester filler on a lot of my patterns, I've never had any problems with mixing it with Epoxy .. As long as it is allowed to cure for at least 24 hours .

I often make patterns out of cheap floor insulation foam .. it will "melt" when in contact with the EC pattern primer, so I "seal it" with a layer of epoxy, leave it to cure for 24 hours, key it with 120 grit, then use polyester filler and EC pattern Coat primer -  If I accidentally sand all the way through to the foam, I just re-seal it with epoxy, then carry on with the Pattern Coat primer ..

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

I've never had any issues. 

I have heard that bondo dont stick to epoxy primer.