I have 5 hp briggs engine that has the gas tank hanging under the carburator on a wood splitter, it hasn't been used for 10 years. I have added gas to the tank to keep it full thinking that I would not have any problem. Now I find the tank wall full of a gritty stuff that needs to be cleaned out.

What should I use to clean the tank out, I have it off the engine. Any help would be appreciated.