I have been looking into paint options such as my epoxy primer thread. My question now is what paint to use invasive the longs if they are opened up. On the mating surface I will use weld thru primer. What about the rest?

I am a fan of POR-15... some people are not.

Go slow with all those welds. one far right, one far left. compressed air to cool it down in between.

might do weld thru primer first, weld then top coat.

IIRC most rust protect paints don't like welding. POR will turn to GOO.

I would spray sem rust shield or cold galvinize spray.. And then once closed up and eveything is painted soaked internally with corrosion x commonly used in aviation to prevent from rusting from the inside out.. I used it on my last six I sold 15 years ago and it just sold again on BAT and looks exactly the same as when I had it.

If it's bare metal, epoxy primer.

It's bare metal treated with ospho

One must remember. What ever you decide, it is a 100% improvement on how they were manufactured. After a 24 hour soaking of OSPHO. For most of us cold galvanizing and Rustoleum paint will be the easiest. Home Depot, Lowes, Ace, etc. If we keep the drain holes clean and we don't leave them sitting out in the rain for years on end it we be all they ever need. IMO.

On the cars I touch. I'll treat the insides of the upper long with Eastwood HD Anti Rust. In addition too the lower rail of the long after all floor panel work is complete. Probably over kill, but will make me feel better.