That is unfortunate. Thankfully, i live in california and we dont see much rust here. I know a lot of guys have us POR15? I think is what it is called, but if your frame is already that far gone i think the only solution is to either cut out the whole section and weld in new pieces, however with rust like that i would think it would be spread all over, if that is the case you might be looking to swap frames which is a decent project in itself.