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Nice truck!
What do you plan on doing to the body? Not sure I'd refinish the "inside" of a truck bed when the outside is old paint.
What make you think after 35 years of no rust that it now needs to be protected in New Mexico? Are you moving to the rust belt with the truck?
Bedliner, DIY stuff is crap, waste of time and effort and a good way to piss of the next owner, after you get so pissed off at yourself for doing it that you sell the truck.
A professional bedliner application won't ruin the "value" at all IMO, although if you're planning on selling an "original" truck, it may not be the next buyers preference. And after you use the bedliner for years it ain't goin to look all pretty and show condition if your plans are to have a valuable orig truck.
Truck is a 1986, so anything you do, the original paint is suspect for peeling off the primer. Surprised it hasn't inside the bed anywhere, actually, so sanding to primer or stripping off is the only right way before paint or bedliner.
If you're actually worried about rust, it will come from the underside and the seams where the old busted up seam sealer is. Best protection from rust $ would be sealing/painting the underside and replace the seam sealer in the bed.
After that, it's up to you, but if I was intent on re-doing the inside of the bed, a quart of quality matching single stage paint and plywood or a bed mat is what I'd do. I'd save the remainder of the $500 or more that it will cost to get it Line-x or Rhino coated unless it becomes a "show truck that does work."
The roll on bedliner, as well as the stuff that comes in spray cans is garbage. It's a thin layer pf paint with some grit in it, wears away the same as paint if not easier, it rubs/scrapes/peels right off. Looks real nice at first, but if you use your truck like a truck (which I can see you do) it will look just as crappy and even worse because now you'll have splotches of black paint over your factory paint and bare metal.
I know a lot of people hate them, but I personally think the plastic bedliners are great for what they do. They protect the bed, but unlike the rubber mats (which also do a great job) they are kind of "slippery" so loads slide right out. They are easy to remove for cleaning, and because they're not attached to anything really and have that corrugated shape they kinda act like a shock absorber and protect against dents too. Also, unlike they rubber mats they seem to drain out better underneath and are less prone to rusty spots IMO (though the rubber ones aren't known to specifically cause a problem in that regard, just my opinion. Yes they do rub the paint off underneath you're right, but that is no different than what you have going on now- if you ever do decide to stop working the truck and restore it (even to original-ish condition halfassed) you're right back at square one where you are now and can decide how you want to paint the bed to make it pretty, versus if the truck is rhino lined now you have to strip all that **** off somehow AND it will be even more dented up beecause instead of a big thick piece of rubber or plastic protecting your bed it was just adhered right to the surface so anything that impacted it also hit the metal right underneath with the same force. If you're worried about it making the bedrails look cheesy, they have two different styles, some wrap over the top of the bedrails (and I agree they never fit right and look pretty cheap) but there are also some that stop just under the rails and tuck in on the inside of the bed
I’d repaint it and as others have suggested a rubber mat or plywood. On second thought forget the plywood, I went to buy 15 sheets of 1/2” it was priced at $80 a sheet,I left empty handed.