Whoops, there is a big crack in your paint. You didn't see it before, but there it is. Was it the Bondo that cracked or was it the application of the paint? How do you find out? We're going to show ...

Don't think so. I'd say just shrinkage of whatever is under the paint. Looks like there may be a layer of sealant under the paint. Rust usually shows some bulging and a red/brown discolouration. What ...