It's one of the basic rules of converting an EV: choose a donor with no rust. Rust could indicate structural weakness, and when you're loading a car down with lead, you want a sturdy frame.
I thought I had managed to follow at least one of the basic rules... until I found a rust bubble under my driver's side rear turn signal lens.
I got some advice on how to fix it, then attacked it with a few flap wheels and a sanding block. I wound up sanding right through the metal and into something ugly on the other side.
Turns out someone else had trouble with rust in this spot before. But instead of actually fixing the problem, he just slapped some bondo on it and figured he was done. Naturally, that didn't cure the problem. And then it became my problem.