for full corrosion protection as per I-car. epoxy, let cure according to manufacturer directions, scuff with 80 grit but don't break thru epoxy, then 2K filler or 2K glazing putty.
if you apply filler over bare metal. filler can absorb water, some kinds don't. or condensation can form under the filler causing rust.
also if filler is applied over uncured epoxy you can have swelling/ shrinkage problems.

perception is 90% of reality