I have a sandy build up on the hot tub surface where there is water? I changed the water in November and used it only a few times - been about 2 months since last use.

Any suggestions?

Sand is a bear to get out of a hot tub. Does the unit have a paper filter and is it clean?

When you say you have a sandy build up on the spa surface, do you mean sand laying on the bottom surfaces or actual sand sticking of feeling like sand on side surfaces as well. If sand is in the bottom you can purchase a product called the fish, which is a vacuum that can suck up the dirt or sand and keep the spa clean of debris. If you don't remove the sand it will start to clog your jets and they will become hard to move along with diverter valves as well.