I had some watercolours in a bureau drawer, not very well protected - and the one on the top had spots of, frankly, spider crap on it: easy to confuse this with foxing of the paper, but it isn't. And, it can be removed: so for those who might find it useful, here's a tip. Don't use, as I once did, a putty rubber - that just removes paint, actually rubs it out (which can sometimes be useful, but that's another story). You can usually remove it much more successfully by wetting the tip of a fine sable brush, and just working away gently at the spot until it dissolves: have a tissue to hand to blot it immediately, because you don't want to leave water lying on the paper - if you do, instead of spider excrement, you'll have little rings where the water wore the paint away. I've employed this method on dark paint, and on very light paint (how come spiders manage to to crap white on dark, and brown on white???) and so far it's always worked. Sometimes it helps - if the spider had a particularly heavy lunch that day - to just scrape the surface of any raised spot with a pen-knife, very lightly, just to remove anything standing proud of the paper (take care not to scuff the surface) . Then go in with the brush. Always better to avoid having to do this in the first place by putting your work in the folders available for that very purpose but - I didn't. And if you didn't either, this works. Oh, and slaughter the perishing spiders, as well.....