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47silver 47silver is offline
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Join Date: Sep 2002
Location: Pittsburgh
Posts: 2,497
1972 911 gas tank rust cleaning

Cleaning a gas tank in the 72. i want to reuse the tank as it is in nice shape. However It was sitting empty for many years.
Here is what I tried and found out while cleaning it.

i first put fastened it to an engine stand, and put in 100 3/8" nuts and spun it a by hand. i removed and installed in a different position and spun it again.
Suggestion: build a rack to hold this with some pivot points and then you can spin and reposition. If i cleaned tanks for a living i would build a motorized device. See you tube.
So this knocked off any loose rust (suprisingly not much) but you need to get all of the nuts out of the tankwhich is not as easy as you think it would be to do.
Figuring that this was goign to take a long time.

I called a rad shop to ask them to boil it and they said the only way they work on a tank is if they boil it, cut it open, clean it out, reweld it and then seal the inside with some type of coating. The tank is in Great exterior shape so i am not inclined to have them cut it open. The cost was about 400 bucks which is worth it but I was not keen on having the tank welded.

I then researched other methods.:
por-15 and derivatives used various chemicals to clean and then seal the tank. There are plenty of reviews on how great these are but other reviews indicate that if the process is not followed exactly and if the tanks is not completly absolutely bone dry and clean then the sealers will not work. The curves in a 72 tank with the indentation for the spare tire etc, to me would create problem areas that unless you filled the tank to the fill spout with cleaner, etcher etc, adn then sealed it by flipping it every xx minutes then it would not be a success and that would be a mess to clean.

Acids: muritatic, Prep and etch (phosphoric) etc, just do not do it for me as one splash and you are blind and with a heavy tank with numerous openings that need sealed, you are risking your health.

Evap o rust, Vinegar (apple cider or white),, i tried these. i only used a couple gallons, i found that they works slowly and that the bare metal flash rusts immediately. Since you have to flip the tank around to to get to every part of the tank, I figured that you need to fill the tank with this material completely, then use other means to stop the flash rust.
Molasses: works like vinegar, i did not try it.

Electrolysis: i am using this method. it i sworking but slower than i thought it should.
use a manual battery charger not an automatic one with 10 amps output.
i put a rubber ring around the gage hole, an old stabilizer bushing cut to fit works nicely.
put in a rebar I also used a 1/4" by 1 inch by 2' long piece of steel that i bent it to fit lengthwise inside the tank. This cannot touch the tank.
I used washing soda (not baking) and mixed 3 cups in a gallon of hot water. Dumped this into the tank.
Filled the tank wihe water to overflow.
connected the postive terminal to the steel piece, the negative to the tank at supply return bolt area. Installed some jumpers from this to other locations of the tank and...
2 hours later rust was slowly oozing out of the gage hole.
Next i tried oxi-clean and that worked even better.
the inside of the tank is getting very clean but it is taking a long time.

my main concern is what to do after it is clean. i read that people put in transmission fluid mixed with marvel mystery oil to coat the inside of the tank so it will not rust.\
I was thinking of u sing the prep and etch to seal it but.. not sure.
1975 911S Targa
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Old 09-12-2018, 08:19 AM
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