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Somehow I've avoided doing this on numerous trucks over the years but am facing a rusty tank issue on a big Unimog tank. I didn't think there was an issue in there but it must be hiding up the sides and/or on the ceiling of the tank because I'm getting deposits in my sediment/prefilter.

A lot of the motorcycle guys seem to like the vinegar route for removing rust. Tons of videos on that subject. I use other chemicals when I do winch and part restorations and really love the POR15 Metal Prep for etching and leaving behind a zinc-phosphate film for paint and preventing further corrosion (even on aluminum) but it would be super expensive to treat a large tank.

The 30% vinegar solution for sale at places like Lowes looks like my best option. Was wondering if anyone has done it? I was thinking of watering it down 2 or 3:1 and letting it sit for 24 hours and checking it.

The issue with that tank is its really hard to shake it around if I want to add some nuts and bolts to help dislodge any rust. How to shake or vibrate a big tank......? Maybe strap it to a cement mixer?

Buy bulk quantities of citric acid powder (Used in cooking so commercial quantities are available) and then make your own solution using water. Go the cement mixer way and throw some shingle in the tank as well. The hotter you can make the citric acid solution, the quicker it will work.

The vinegar solution will work as well, the stronger the solution, the quicker it will work along with the temperature of the solution.

Another option is to use cheap "Fizzy drink" e.g. own brand cola, or similar. They all have citric acid in them which is the active ingredient.

John

Vince, have you considered a purpose made rust remover like EvapoRust? I have found it very effective on some tools but not a large tank as you describe. It works a little differently than the acids.

Possibly another alternative is to bring the tank to someone who
specializes in fuel tank re-lining. I went that route when I was
rebuilding my 712 which had a bad case of Red death. They managed
to get the inside to bare metal then re-lined it. Came out nice.

Thanks for the ideas. The budget is the main factor for me. So many small resto shops use the vinegar technique so I'm trying that one first I suppose. I think I'd have to buy too much of the speciality stuff to cover enough interior space of the tank due to the 40gal size.

The businesses that restore tanks are too far away from me and the mog tank is too big to ship to make it cost effective although that would be great. I've seen some tanks done that way and it looks like most of them cut holes in the tanks to fit sandblasting nozzles into then carefully weld back in patches. Luckily I have a very light corrosion issue so it wont take much.

I screwed up and forgot a before pic but here's one showing the container I was pouring the vinegar into after letting it sit in the tank for two nights, which actually looks a lot like the tank did.

I put in two gallons of the extra strong commercial vinegar like you buy at Home Depot. I would rotate the tank and let it sit for a half day or more in each position. I also treated it with a gallon of strong degreaser to help wash out everything else, which there was a lot of.
Then I took this after several rinses with a garden hose.... it worked amazingly well as you can see! There was rust all over that metal before. I did one more treatment with the "POR15 Metal Prep" that I use for winch parts (I recycled that, which you can do many times). This stuff leaves a zinc phosphate coating to help prevent flash rusting. I immediately put my heat gun in the fill hole and dried it out fully for about 30 minutes. Ideally I'd be able to coat the entire tank with sealer but the price on that stuff is really high since I'd probably need two quarts. (one quart treats about a 25g tank) The diesel fuel and my NV climate should keep it from rusting again.