Fender channel help

I did a similar thing.

I had a good rust free fender. I had it chemically stripped / rust removal

I then sealed up the support and poured a 2 part epoxy paint into the support.

Moved the fender around to coat all surfaces. Then sprayed all the rest of the fender.

Word of advice is to be sure to put panel sealer in all the crevices / joints between the parts that make up the fender, BEFORE you paint the topcoats