One of the worst garages to store anything in is a garage with an corrugated asbestos roof, those bad boys sweat and drip condensation so badly my Fazer ended up with mould spots all over it a few years ago, even thought it was covered by a cotton sheet (winter storage) The sheet was almost wet one day when I went in to check the bike.
My Father-in- Laws dark green Astra got white ring marks all over the bonnet, roof and trunk lid from the asbestos roof in his garage, his car was in there for about 4 months after he had a hip operation. He had the car professionally buffed but the marks did not completely come out, it was as if something had eaten into the gel coat.