I didn't wetsand mine at all on my first grill. On my second grill I wetsanded it once I had the gap filled in 100% to my liking.

Yeah that could be the issue. I think after I did the first fill of bondo I waited for it to cure overnight before I started sanding. Then each time after that I'd wait at least a couple hours. With the filler you don't have to wait as long.

That is the material I used. When I did mine it didn't crack when I sanded it but it exposed tiny pin holes in the bondo. I had to use the puddy filler and then sand with a very fine sand paper to get rid of the pin holes. It can take a while and a lot of patience to get it right (and even then it wasn't 100% perfect). Hope this helps.