The best thing to use on bumpers is epoxy primer.
The next best thing would be a 2K primer, I use it most when
needing to fill scratches. It sands easier than the epoxy but epoxy
can also fill small scratches.
With either one you don't need an additional sealer, the primer is
ready for paint and both of them seal pretty well.
In either case I would remove all the rattle can primer, it's the weak link.