How do you remove rabbit pee stains from a litter tray?!?

Okay, so I have this enormous white male Angora rabbit (not neutered, but that's because I live in China and the vet's are only for cats and dogs, not for rabbits) and he lives in a hutch. On the bottom is a retractable litter tray, and his pee has made these yellowish-brown stains on the bottom (huge ones, practically indelible). They're dry and I looked online at another Yahoo! Answer which said I should use white vinegar so I poured some on the tray and tried scrubbing it off with a very hard brush. It's working, but REALLY slowly and the stains are flaking off really small, so I was wondering if anyone could help me and tell me another way to get them off? Remember I live in China so something that is available here would be the most helpful.