If the fenders are aftermarket, the primer needs to be checked for solubility by wetting a wipe with reducer and rubbing the primer. If the reducer takes it off, the primer should be completely removed.
If the primer is not soluble, a very good scuffing with a red pad (no more shiny spots) followed by epoxy is best, as BK says. 2K urethane primer is also acceptable, if there are no bare metal spots.