I put two vehicles outside for storage a couple years ago; one is a 91 GMC truck, the other the Kelmark GT. I used the common plastic tarps with grommets they have around here to cover them. Bad Idea! Anywhere the plastic tarp was pulled tight to the vehicle's surface, the paint surface was utterly ruined. On the GMC the clear-coat lifted right off the body and on the Kelmark, the stain went even deeper.
In retrospect, I'm not surprised and I think I know (not sure) how it happens - the tarp gets wet and is not fully waterproof, then some chemical in the tarp merges with the clearcoat or paint. Then it dries and releases and lifts the clearcoat off and leaves a stain of its own color. Maybe the fact that the tarps froze during the cycle has something to do with it. Dunno 'bout that part.
One more Stupid Point for johnboy. Hope nobody else makes this mistake.
(FWIW, the two good Bugs are stored with full cotton body cloths under the tarps. No problem with them!)