"dadiOH" wrote in :

As Lew said,
Bondo is polyester resin filled with (mostly) talc. To my knowledge,
cured polyester doesn't "suck up water"

Actually, polyester is notorious for doing exactly that.
Google for "polyester blistering" and you'll find 1000s
of articles on the problem.

As for the original problem, I'd agree with those who
suggest cutting off the affected part, and installing
some sort of plinth. There's really no way to treat
rot other than replacement.

John