I recently restored a car where a replacement fuel tank, and fuel sender were not available. I tried muratic acid, which removed the rust, but did nothing for removal of the tar-like gunk that coated most of the inside of the fuel tank.
So I tried an industrial strength grease remover used for cleaning food service kitchens. The product I found locally is called Dynamo-X, made in Helena Montana. It can be diluted 50/50 or weaker. Sells for about $17 per gallon, and is biodegradeable.
I mixed 5 gallons of the dynamo with 7 gallons water. Let the tank sit for 10 days, stirred it once a day. The tank came out spotless inside, no further work beyond pressure washing, rinse with clean water, let dry in the sun, an slosh the inside with 3 bottles of Heet moisture remover.
I then soaked the "frozen" fuel sender unit overnite in the "dirty" dynamo solution. Very lite scraping removed all rust and other gunk. The sender checked out perfectly after that and was able to be reused, and I had thought it was a goner.
Hope this is of help to someone. It sure saved the job I was on.
