Gator, If you scuff the Urethane, you might be just fine to apply the filler
but if you suspect that the primer has not got good adhesion then that
will be a weak link.
I would remove the primer to bare metal.
Apply filler "Not Bondo" I use Metal 2 Metal as my filler of choice !
Top coat this once shaped and smooth with the primer of your choice
Epoxy, or regular 2k.
If you do a search on the board here, there were some lengthy discussions
on this subject..
Try the search feature.