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I removed my sending unit when cleaning the varnish out of the bottom of the tank with acetone. My car sat from 1974 to 20167!!!!. Varnish was almost 1/2 inch thick in the bottom. If yours has a thin layer of varnish, you may be fine just soaking with acetone and then draining.

I sprayed the sending unit down with carb cleaner. Ironically, the main ingredient of carb cleaner is..... acetone.
I don't think it would hurt a thing to soak it in acetone, but cannot guarantee it.
When i put it back together, the gas gauge did not work. I put in a tank of 100LL aviation fuel.
 

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Opel Rallier since 1977
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I don't think a flush with acetone will cause any noticeable damage. I am not planning to soak with a full tanks of the stuff for days. just rinse out a couple times

and float if damaged can be changed easly with out removing tank. IF TANK WAS OUT OF CAR it would get a different approach
If you are putting acetone down the fill hose, I disagree that there will be no damage. The fill hoses tend to be old and already dried out' acetone will just make that worse. I don't understand messing with something that is critical to safety.

And once it dissolves any gum, how will that combined acetone and gum (whatever it becomes) be gotten all out? I am not seeing how that will be accomplished. If it is true that acetone is a primary component of carb cleaner, then any significant amount will make the engine run poorly, and I sure would not want to draw the resulting gunk, whatever it is, into the carb in any significant amount.

I agree with getting the gum out being a good thing. But the gradual running rinse described by Autoholic is the ticket IMHO. Or pull the tank.
 

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I recently restored a 88 Saab Spg and the gas tank had 1/2 a tank of gas that had broken down. I poured 1/2 gal of acetone in it to clean it out. The only redeeming factor in this equation was the Saabs gas tank is heavy gage plastic, and where the fuel pumps go, was open enough I could reach in and wipe the tank clean. I huffed enough fumes to kill a bunch of brain cells between the old varnish and the acetone. My son was bitching "pull the tank, pull the tank" as he had seen me do with the Gt. Nope I carried on, and that 4 days had to be the most miserable days I spent this summer. Just pull the tank, you'll appreciate it later, It's not that hard.
 

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Maybe my comment it's late but I used before apple cider vinegar with really good results, just let it sit for a couple of days and drain. Use as much as needed to cover the dirty or surface rusted area. It will work with the tank in place, no need to add rocks or chains. My two cents, good luck!
 

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I just did a full cleaning (tank out of the car) and "Super Clean" purple degreaser was very effective at getting the sticky, old gas, sludge out. I left it in overnight and it took care of everything but the rust. It's at most auto parts stores. If you still have the original fuel filter sock in the tank, I would definitely recommend pulling it out as it would probably gum up with whatever's being cleaned out.

If you use an acid, (muriatic acid worked well for me) it will get rid of the rust, but you'll need to immediately coat the new bare metal with something else, or it will start rusting again right away.
 

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Detritus Maximus
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That’s what I’d do on a runner if I didn’t want to pull the tank. Add Techron to the tank, use clear fuel filters, and keep changing them regularly until the filters stop capturing dirt. It would be a good idea to have the filter easily accessible and carry a few spares and tools to change them in case it needs changing on the road.
I tried that with mine when I first got it. Worked great. Worked so well I got to the point I couldn't go a mile before it would clog the clear filter I installed in the fuel line near the tank.
The problem is that many things will loosen some of the gunk for a short time and it will tend to move towards the fuel line. It will clog the sock or hole to the point of nothing coming out. Then you have however much solvent in the tank and no way to get it out.
If you use just a little solvent, you get just a little bit of gunk out.
The key is to get the stuff loose and dilute it slot.
What would be great is a vibrating motor you could attach to the tank, an outlet hose going to a bucket with some sort of filter media to trap the gunk but let the solvent flow and a pump to recirculate the solvent back into the tank with something like a shower head with a wide spray pattern.
It might work. And then there is the baffle in the tank...
 
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