I made myself a gas tank cleaning kit. Find a bunch of SAE (nobody needs that shit) nuts or bolts. And count them. Throw them in the tank with bad gas. I keep my bad gas exactly for this reason. Or if it’s really bad go to Home Depot and get Muriatic Acid. It’s in the pools section, it’s cheap and lasts a long time. And will most def clean that tank. You want to cut it with some water though. Anyway fill it up. Throw the bolts in. Shake it around a bunch. And dump it out. Drain it. Count your bolts to make sure nothing is wedged in there and you got all of em. Flush it with bad gas. Then flush it with good gas, just enough to work. Then you should be good. If you’re worried about future rusting just make sure to keep the tank full if you’re storing it for a season. Honestly I almost always get away with just using bad gas and bolts. And not having to do the acid. Good luck Hope it works out.