I wonder what they're having the bows re-haired with - it's fairly inexpensive here.

I think the rosin tends to stick together and build up into a gritty-sounding ...er, buildup. It does this on strings too. I know it gets to a point where my bow sounds scratchy, and cleaning it returns it to normal. I've scraped the gunk off too, but I wouldn't be surprised if this was not a very good thing to do to bow hair.