I have had luck with brake cleaner or starting fluid, but it will melt your gasket. Spray it around the edge like WD40 on a bolt and wait a day or three. Clean up thoroughly with dry paper towels.
But try the ice first.
True, sand does not wear out. It does accumulate oils and residue on its surfaces that will not rinse away with water alone, leading to clumping, channeling, and clogging. When allowed to dry it can stick so tight it is nearly solid, and the pump will blow channels through it leaving you with flow but virtually no filtration. I see this most on hotel, apartment, and HOA pools; lots of suntan lotion, make-up, heavy bather loads, etc. and generally bad maintenance being performed by a guy who's primary job is mowing grass. But I have ran into it in private pools many times as well.