ORIGINAL: bbb18c1
Fiberglass tape for drywall and shoe goo works really well.
Stick the tape on the inside of the body where the crack is and then spread shoe goo over it.
You can also use goop instead of shoe goo.....I think its a bit cheaper and its the same stuff.
+1 on the shoe goo. It works wonders! Drill a small hole where the crack is so it can prevent it from spreading. I have seen a few bodies with bondo on them, but its time for a new body when it comes down to using bondo.