I used Bondo to re-contour a GG1 body a couple decades ago. It is still absolutely beautiful. I did prime and sand, as suggested above. I think JB Kwik is the right stuff for a small build up.
When I was a kid, I thought body work absolutely had to be hammer weld and lead. Later on I got lazy, and fixed some stuff on my Mustangs with Bondo. That was in the 1970s. The Bondo is still there, no cracks.
I also used it in 1966 on my aircraft cowl, which was not perfectly smooth, since I hammered th sheet metal into concrete form blocks. The cowl is a little worn from 4000 hours of flying, but the contours are still crack- free.
