At this stage you need something "liquid" that can be built up in layers ans sanded smooth. Repeated coats of primer, Mr. Surfacer, or thinned putty would be my choices. Mr Surfacer is a jar of well, thinned putty, LOL. It can fill seams quite well.
Another trick I have used is typewriter correction fluid. "White Out" is easy to apply, dries fast, and sands beautfully. It will take any kind of paint and not be affected.
G