Sorry to sound like a d*ck but I'm w/ George Downs. You can't beat rust. You can slow it, cut out around it and replace, .... but if it comes from the salt-belt you'll lose eventually. I've got a neat old Lancia in my shop. 20K miles; rusted through underneath beyond cost-effective repair.

I'm building a 240 winter driver as we speak. It's a Cleveland OH car and there's rust in all the seams. Worse for me is the frozen and rusted mechanicals: OD solenoid, trans lines, calipers, etc. Trailing arms are rusted through - the spring split one.

Why am I bothering? It's a low-miles car with good maintenance. My plan is to get 3-4 years of winter driving out of it - and keep my good ones in the garage Nov-April.

And I'm going to try to find 2-3 240 wagons in AZ/ W-TX, OK, ... and bring them back here. I've no end of good mechanicals but all of our cars have been sent to the barn for rust.

So, try POR-15 if you like but you'd be better off in the long run to sell it and find a rust-free Western car; or use the mechanicals in one of those.
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240s: 2 drivers & parts cars