To clean mine I drained it and then blew it out with compressed air. Then used acid... Depending on what cane out I would used CLR and either a bunch of nuts and bolts or a small length if chain. Then shook the crap out of it knocking all the stuff off the inside. Thoroughly rinsed with water, then compressed air again, and then I let it dry for the day. Next day cover the inside in POR-15. Good as new!
Edit: also, if the gas was sitting in the tank, it was sitting in the lines. If you're not replacing them, make sure you blow them out.