Your best bet is to take back down to bare steel and sandblast any rust where the bondo was. (Its the rust under the bondo that causes the bubble to occur).


1937 Chevy (converted to 4x4)
1942 Chevy G506 1.5 ton 4x4
1958 Mack B model (on Chevy 1 ton 4x4 frame)
1969 Dodge Charger
1987 K30 Crewcab