I just cleaned one that had been sitting for 3 years. Lacquer thinner I found to be the best way to clean it. I never put any bolts or rocks in tank to help clean it. I just let it set for 4 weeks turning it every day until it worked the crud off. Scrub brush helped also. But rockets idea of just getting a new one is also a good idea just have to spray it with undercoating, so outside wont rust. Hope this helps...