manually cleaning the inside of a gas tank

What is your question?

Hello, I drained the gas tank of an '86 Subaru Brat that has been sitting for years and old,thick,dirty,sludgy gas(?) came out. I removed it from the car and want to clean the inside real good before putting it back on. Wondering if there is a solution of some kind that I can use. Any thoughts? Thanks for any advice or tips.