I have an '85 that lived its life in GA before I got my grubby hands on it. It's got this fibrous, kinda organic-like rubbery/leathery undercoating that's kind of thick underneath it. I like it. No rust on the beast, save for a spot that was caused by a plugged up trunk-hole. These things are supposed to have undercoating, from my limited feeble understanding. Mine may have had supplemental rustproofing applied, when I peeled some back whilst treating the lil spot I found from underneath it almost looked like leather on the inside, but it's black on the outside. Weird stuff.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)