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47silver 09-12-2018 09:19 AM

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.

mepstein 09-12-2018 02:42 PM

I've done a couple tanks with apple cider vinegar and then coat with MMM. Takes a couple days to a week but works every time. It's strong enough that you can dilute with some water to fill the tank. I like it because if I spill in my garage, dog drinks a puddle, he won't die. I can pour it out in my woods. Then keep the tank filled with fresh gas and a tablespoon of MMM.

72911 09-12-2018 04:50 PM

For some reason, I had this saved in my favs. I have not used it but maybe you can let us know if it does.

Kreem | Fuel Tank Liner

Flat6pac 09-12-2018 05:02 PM

With out the swirl pot the tanks can be cleaned at a radiator shop, dipped, cleaned, and coated.

47silver 09-14-2018 05:46 PM

was using rebars for the positive side and although it works i deteriorates , needs cleaned and the water is rusty...
i bought carbon rods 1/" x 12" and started using them yesterday and they are not dirty, the water is black though but the tank is almost completed clean of rust.

one more day of this and i think it will be perfect.

i read where i should put some isopropyl or denatured alcohol to absorb any water and it should be good to go.
when i get a chance i will post some photos.

Trackrash 09-14-2018 06:02 PM

Yea , the tank will rust very quickly when you are done.

I coated the inside of mine with WD-40, which worked well.

47silver 09-14-2018 07:34 PM

prep and etch
would it make sense to use something like prep and etch or the por 15 metal ready after this process? i do not want to seal it with a liner but was reading that these leave a bonded coating and that the por-15 product is a zinc phosphate coating. others use a mix of auto transmission fluid and marvel to coat the inside.
i wont have gas in it for a week or so .
any advice will or experiences are appeciated.

RedCoupe 09-14-2018 08:46 PM


Originally Posted by mepstein (Post 10179356)
I've done a couple tanks with apple cider vinegar and then coat with MMM. Takes a couple days to a week but works every time. It's strong enough that you can dilute with some water to fill the tank. I like it because if I spill in my garage, dog drinks a puddle, he won't die. I can pour it out in my woods. Then keep the tank filled with fresh gas and a tablespoon of MMM.

What is MMM?

juanbenae 09-14-2018 09:08 PM

I was surprised that you could get a new repro dansk tank for <$500. if COA was not the goal it seems silly to not just get a new repro and keep your old one for rehab another time.

look at what a repro oil tank costs and the fuel tanks is cheap.

florio 09-15-2018 02:53 PM

I have done a tank with this kit: it comes with a degreaser, de-ruster, and a special two-component sealant for the inside. The result was excellent, all the rust was gone after the first step. I dried it with some acetone to be sure there was no water left inside. The coating came out well and they brand it as military grade, whatever that means.

I'm on my way to treating my 911 tank with this as well (no plastic swirl tank inside)

Sorry it's in french, no affiliation

47silver 09-18-2018 01:39 PM

tank done
As you can see the tank turned out very nice.
this is not for everyone but here is what i did
5 years ago i removed it from the car, the tank was empty and had sat since 1994.
the outside was decent and the inside was not too bad.
Toclean off the outside I used a cup wire wheel on an angle grinder and made quick work of the coating that was on the exterior. It is easier than you think.

I cleaned the inside (which was not too rusty with apple cider vinegar), That works but takes a week or so. I neutralized it with water and baking soda. After that i had it painted using upol gravitex that a friend color matched some samples i saved by mixing some blue into the gray/white bottles and had it sprayed.
The tank came out great and i put it on the shelf. I did not realize it would take about 5 years to reinstall it so when i looked into the tank (5 years later) it it was coated with surface rust. Probably caused by not getting all of the water out.

since the paint was nice and the tank was sound I decided to just get the rust out.
first i tried nuts and metal on an engine stand and twirled it around. Very ineffective and not worth the trouble.
Chemicals and i dont get along and I researched tank cleaner/sealers. All aaid that they were the solution but the negativfe reviews about them peeling turned me off.

I called radiator places 2 of them would do it but they cut open the tank, clean it out, reweld it then seal it with some substance.
then i tried electrolysis
after i switched to carbon rods it took perhaps 2 days (i let it run another day) and the rust was gone. I used a solution of marine clean and hot water and cleaned out the graphiite that was in the tank. I coated it with a mix of atf and marvel and it to me is like new.
As an aside I would have bought the Porsche tank if could not get it clean. After the paint job etc what is another 1500 dollars.

nickelplated5s 09-18-2018 03:17 PM

Been there, done that. Get the Dansk from our host.

cmcfaul 09-19-2018 11:59 AM

I did the whole reseal the gas tank thing on my 73 E. There is a kit you buy (POR...) and follow the instructions. Had I to do over again, would have bought a new, aftermarket tank.


47silver 09-19-2018 08:58 PM

i researched them and found that they can work but only on gas tanks that do not have a lot of curves. the 911 has too many curves and i think it would take 3 Por-15 kits to make sure everything was clean and all surfaces evenly coated. Also they emphasize that with por-15 the tank has to completely absolutely dry before the sealer is dumped into it. You either have to use a couple gallons of denatured alcohol or acetone and then you have to dry it in the sun with a sweeper hose or hair dryer blowing into it for hours.
That is why i went with electrolysis. No nasty chemicals to worry about (Por-15 is very nasty stuff) and turn it on and leave it alone. It worked and using a scope i did not see any rust, but time will tell, if i find that the gas filter is clogged after 100 miles i will buy a new tank.

BoxsterGT 09-21-2018 01:24 PM


One issue with the Dansk reproduction tank is that it does not have a pre-filter before the pump as the original.

Many folks feel this is necessary to protect the expensive fuel pump.

If interested, I offer a SS screened Pre-Filter with the correct fittings & supply hose and Norma clamps....

It does require relocating the pump but that is not difficult to do.

Len dot Cummings at Verizon dot net


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