Give it a good rinse and knock loose what you can with a power washer or hose. Soak in citric acid for a few days and agitating time to time. Im confident in you fellas abilities to agitate! 😁

Give it a few rinses with a pressure washer and splash around some baking soda inside when you’re done. I did my Farmall 560 this way and did great with only issue being from the baffle divider.

Keep safe!

There is a company here in town that does acid dipping, American Metal Cleaning, they can dip whole car bodies and frames, they come out shiny and clean, same with fuel tanks. I have taken my starting engine gas tanks and other tanks there, just wash the tank with soap and water then set it on the shop woodstove to dry out then use POR 15 sealant.
There contact info is on the top lefthand corner of the web page.
AMERICAN METAL CLEANING, INC. (americanmetalcleaninginc.com)

I did my D2 pony tank that way--nuts,bolts.course sand and strapped it to my lawn mower rear tire--came out pretty clean

The Napa numbers for a D4 6U that I needed filters for are Filter #3825 at about $25 each, and the gaskets are listed as "Napa gas filter" but they are the correct gasket #5635 I dont remember the price but they were very in-expensive.. They told me that the D2 takes the same fuel filter as the D4, but with different gaskets. Good Luck Grant.

i hadnt thought of acid dipping or hot tanking them that would work very well. only thing is it would need sealing with a oil etc immediately or it will flash rust. to remove any oil is easy a bit of wax and grease remover or thinners slosh it round and let dry before treating with tank sealer.

They have a rust preventive after they dip them, thats why they said to use soap and water to clean the tank. Then heat the tank to dry it out then seal it.