Fiberglass with a thin putty coat... wont shrink. will flex so it wont pull away with body flex on a seam... Lead, I have used it before, and successfully, and 'Bondo', by whatever plastic filler name, does have its place. It (to my knowledge) doesn't so much shrink as it is rigid and therefore will crack or suck up with body flex, especially at a seam. For a drywaller, bondo should be really easy for you to work with, just act like the final wall is a 'flat' wall with no texture, so your mudd must disappear seamlessly, and you will have little sanding. add a thin putty coat, 5000 grit wet sand after prime; paint; done.
I'm gonna have to check out that Metal to Metal... new to me, but haven't been in a body shop (not counting my garage) in quite a while./
Laterz,
Gator
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