What’s the year ?
If that tank is in solid condition structurally then I would make up a tip for your compressor with maybe a 16 inch long piece of flexible copper tube and pinch the tip a little for better pressure. Angle the copper as needed to get inside. Stick it in the tank and blow the sh** out of it. Hit all sides and the bottom and creases as best possible. Then fill it half way with boiling hot water, add Dawn kitchen dishwashing soap put the cap on and shake the tank good. Spill it out and rinse it all out good with a hose. Blow it out again to get the rest of the junk and water out. Then hit it with a hair dryer on hot so all the water inside dries up and creates no rust. Spray the inside all around with WD 40 and let it sit a day.
I would put a quarter tank of gasoline in it put the cap on and swish it around and dump it into a clean bucket and see if you get any more flakes or dirt.