I think the camo paint and cosmoline the military used did a lot to ward off rust...wish GM used something as effective!..
Rust & rot is always worse than it looks...as the saying goes,once you start peeling the onion --..

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My '85 K10 diesel Burb was practically mint in 2009 when I got it--it spent only a year in VT and MA before I got it,it came from Washington state originally,and whoever had it undercoated it thick when it was close to new,the undersides were mint,the frame still had the oem black paint on it..the floors under the carpet had no rust and were covered with what looks and feels like motor oil..
After it sitting in my yard since '09 ,the thing now has severe rocker cancer,I put my hands thru all the rockers from front to rear,and all the door bottoms and rear quarters,rear wheel arches are bubbling up and will be holes if I start poking at them,and that was caused mostly by it being completely buried under a huge snowdrift for 2+ months in 2015,when we got 12 feet of snow between the end of January and March..when it melted it all went in the doors and likely soaked the carpets--I'm afraid to look under them now !..
The frame rails ,springs and floors still look decent underneath though,despite it being parked on grass..
I should have parked it in my garage instead of all my garden tractors and other "stuff"--too late now though..its so big ,practically nothing else will fit in there if I had put it inside..
I had planned to get its 700R4 fixed or replaced and put it right on the road back then,but life had other plans--my back was getting worse,my mom was very ill,until 2013 when she passed away,by then I was nearly broke,so I haven't been able to do much with it..
About all I did to it was install a Fisher plow frame and a Diamond plow blade on it and the pump & joystick,if nothing else it may end up being a spare "yard truck" to plow my driveway...
