I buried a couple back in 99 and when I moved I dug 'em up. I used a 6" pvc sewer pipe and filled it with nitrogen and sealed both ends. I actually buried them cause I was in a high crime area and these were extras. Anyway they were in perfect condition when I dug "em up.
I new a guy (ex-ranger Vietnam) that lived in the mountains of East Tennessee and had caches all over the mountains. Strange dude but likable enough.
The tractor dealers carry a product developed by John Deere for use when they shipped equipment overseas. You spray it on and it forms a very nice coating that's impervious to practically everything.