Basically soda crystals. but sometimes have 20 percent Al oxide. Plus the media has a substance that retards H2O. It takes a considerable air compressor to provide the 12-15 cfm at continuous 80 psi. None the less, it is really a quick solution. Just let the bottom dry completely. I pulled my rudder before the haul out, placed it in the bung of a 55 gal plastic barrel. and put 30 gallons of H2O so it would get blown over during the process. The guy that did the job thought I was a little rad.