Currently I am trying a new-era filler, Marson Platinum Plus.

It, like many new fillers is "lightweight" and formulated to also work over paint. If you plan to primer, before you bodywork, I would recommend this one. It sands very easily, and will adhere to a sanded acid wash primer, or epoxy primer....however, I would bury it in a 2k primer, since it seems very porous, and could cause shinkage.

My old dependable filler is Marson Whitefill. It is very flexible, but must go over bare metal or fiberglass. I've used this one for 20 years with no failures. I always bury it in 2K primer, also.

I use Marhyde's Ultimate Primer. It is a little less expensive than some, and has served me well for 10 years. I always guide coat and block it, then fill and sand the minor problems with F/E Eurosoft, before sealer and color coats.