If the e-coat passes the lacquer thinner test, then you may scuff sand it very well with Scotchbrite, wipe with SPI #700, and apply two coats of SPI epoxy primer. After that is dry enough, filler may be applied to the epoxy without sanding for up to seven days.
Some people do not trust e-coat, depending on the maker of the new part, the e-coat can either be very good or very bad. If there is any doubt, sand the e-coat off with 80 grit, wipe with SPI #700, and apply SPI epoxy. After the epoxy is dry enough, filler may be applied to the epoxy without sanding for up to seven days.