Paint basics.

Thats an etching wash, Not etch primer. Not really needed if your going to use an epoxy. Most epoxys will say DTM on them which is direct to metal. We use to use the wash but the fact that right after you would wipe the panel down it would flash with rust kinda scared us off.

Your most likely Ok with your filler over it.

How ever there is a product that SEM makes called Rust-Mort. Its said to stop rust and you would use it on a panel with pits in it. Wipe it down, let it sit for 24hrs, then primer over it. I have 3 cars coming back to the shop with paint peeling off were we used it. Not cool.