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It's not the salt. It's the water that gets trapped underneath
It's the combination of both that really does the damage. The chemical reaction between regular water and steel is relatively slow. It gets accelerated greatly with the addition of salt. Add in the heat from a black plastic liner baking in the sun and the chemical reaction happens even faster. But because of that heat, they usually don't stay wet underneath for long if they're clean underneath, let dirt/leaves/trash build up and hold the moisture and they rot.
My father bought a truck with a drop in liner off a guy he knew that religiously pulled it out every spring and rinsed the bed out. The guy also washed the truck weekly. It only had minor surface rust where it had rubbed through the paint after 20+ years of New England winters and there was no rust under the truck. Like just about everything else in life, you keep it clean, it's gonna last longer.
Drain holes ... and not just two or three. Across front, front corners, and a few along the sides both ahead of and behind the fender wells. Not little ones, but 5/8" or even 3/4" ... before putting a liner in. A coat of the $50 gallon of Iron Armor from HF (brush it in all of floor panel then toss brush away) too.
It's not the salt. It's the water that gets trapped underneath
With any dirt and the vibration of the liner is like sand paper to sand down the paint & primer.
Look back a few posts and someone said it took paint off the top rails and you could see that.
It sounds like the OP uses his truck other than taking it to shows so I vote for spray in liner.
And for the hatters I have a metal ribbed floor in my 81 flare side and the floor from the parts truck was lined so I left it.
I am not afraid to put anything on the floor, walls yes, and if I had the wood floor it would get messed up and need treating every year.
Dave ----
For my vote is a LIne X product. Much better here in PA. To much road salt damage otherwise. Bed liners here cause the bed to rot faster. Unless
like said above you remove it and clean after use.
