Cleaned tank out twice using a half gallon of Klean Strip phosphoric acid and water. Quite possibly the nastiest tank I've ever run across. Went to drain the tank after the first cleaning and when I pulled the ziploc bag and hose clamp off the fuel sensor opening I got nothing. Looked closer and the opening was PACKED with a black silt like substance. Had to use screwdriver to it knock loose. Keep in mind that this opening has a diameter of roughly 1.25" so low flow, let alone no flow, should've never been a problem.
All manner of shit continued to come out via flushing. Did a second round of cleaning with the remaining half gallon of phosphoric and continued to get a lot of crud out. Fuel outlet was particularly stubborn though. Possibly the worst location/design of a fuel outlet I can think of. Had to unclog that area with fuzzy sticks from the craft section since they don't carry pipe cleaners in the tobacco section anymore. Pfft. Finally got good flow after 10 or so fuzzy sticks. Tank now good but might've just bought nice used tank if I knew how much shit was in it from the get go.
“The real cycle you're working on is a cycle called yourself.”
― Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance