small clean (fishtank rocks work good)rocks or ball bearings and shake to get the loose heavy stuff off. Then muratic acid there is stuff called Kreem that comes in 2 parts the first part is the muratic acid then the 2 nd part is a coating for the inside of the tank. Be SURE you get the inside of the tank very clean before you use the coating or it will peel off the inside surface and cause even more problems.
http://www.jcwhitney.com/autoparts/Brows...-8025067/VT-schhttp://motorcyclecity.com/Parts/tank-repair.htm It's also available in many many cycle and ATV shops as well as many many places online those links are just to give examples.
