Why does body filler fail, Bondo?

[30westrob]

I have observed a large % of cars repaired using filler fail. Common failure is shrinking or cracks. Are the failures due to poor quality fillers or poor application. Are there proper ways to use filler, resulting in long lasting repairs. Rob

[rowingone]

Cracking, chipping, shrinking are all tied to putting the filler on too thick. Filler should be used for final fill only, after the proper body work is completed.

[Dpmulvan]

Originally Posted by 30westrob

I have observed a large % of cars repaired using filler fail. Common failure is shrinking or cracks. Are the failures due to poor quality fillers or poor application. Are there proper ways to use filler, resulting in long lasting repairs. Rob

Poor Product, Poor ratios, mixing on cardboard, poor panel prep improper cure times before paint and applying too thick.