Find a place that does oil undercoating. There are quite a few around here, and I would assume in any rust-belt area.
They shoot the undercarraige and inner panels with usually a bar and chain type oil (super sticky). THey drill little holes in the doors and shoot it in there as well. Good for at least a year, and the truck will not rust.
Makes for a nice job to say pull trannys and the like. Bolts turn easy, but you had better like getting dirty..or the feel of a hazmat suit.
It costs about 75 bucks the first time, and around 40 a year after. If you want to do it yourself and save coin, take your compressor, hook up a siphon tube with a spray tip and toss it in your used motor oil, or buy some aerosol cans of bar and chain lube and pull you inner rear panels of the runner. That will give you access to the inner quarters, where the rust will start.
I owned an 85 4Runner a while back that had a bit of rust. I fixed some of it, but not all.
I pulled the entire interior of the truck apart - down to bare metal. Found and fixed any rust spots, cleaned and sanded the rest. Then (following the directions), I slathered on the POR-15. Did this on the floor and all outside panels, inside doors/fenders, etc.
The rear fenders were pretty much rusted through at the seam. I just kept slathering th stuff in there until the outter fender was again attached to the inner fender. Once that dried, I slathered even more in the fenders until I "shaped" the seam well enough to keep the water from standing in teh seems
THEN, I went underneath. Pulled the gas tank, cleaned up the entire undercarriage (firewall back) and did the same thing. I used a brush to put the stuff on thick in the areas I was most concerned about, then I sprayed everything else using an old paint gun.
Drove it all winter that year w/ no signs of rust coming back. My friend has the truck now and it looks as good as it did when I sold it to him.... almost 3 years ago.
Now - That won't solve your problem. One of the rust-areas that you will face will be the drip-rail around the top of the truck. I bought an 85 that has spent 10 years in Hawaii and it had huge rust holes around the drip-rail. I don't have a good solution for you on that one.
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