We had exactly the same problem when we were at Clippesby last year (with our Bear Lake too).
We asked a few folk whilst we were there what they did / recommended and the general consensus seemed to be either wipe it off with a cloth and hot water, or let it dry and chip it off.
Thankfully, we were there in the middle of summer so the said offending articles dried quite quickly and we could just chip most of it off before we packed up. The rest, we cleaned off with a cloth and hot water (especially those that had managed to hit a window!)
------------- Junk: stuff we throw away... Stuff: junk we keep...