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I had pretty a good result cleaning out my rusty CB200 tank with cleaning vinegar, salt and rocks. So I'd thought I'd share it here.

I used 6 liters of cleaning vinegar and about 500 grams of table salt. Table salt is supposed to amplify and jack up the acidity of the vinegar. I then filled up the rest of the tank with about 3 liters of small pebble stones and some water. I left the whole thing brewing for about a week and every day I picked the tank up off the frame and I shook it vigorously in various directions for about 5 minutes. Please don't forget to put a piece of plastic bag under the fuel cap to prevent leaking when you shake it in the upside down position. After a day or two my tank looked 90% clean but since it is impossible to see the bottom of the tank, I decided to leave it in for a bit longer.

After two days:
IMG_20180829_190652.jpg

After 7 days it looked completely clean but since I had no time to work on the bike I decided to leave it standing for another week. The experts online all say that it only dissolves the rust and leaves metal and paint alone so I thought that it would be safe. The experts also said that it will corrode aluminum and after 9 days I found out they were right when I was at work and received a WhatsApp message that there was a large smelly pool below my CB200. Rather worried I drove home and assessed the situation. It turned out it that the fluid completely corroded through the petcock. Fortunately, the petcock was already written off because it was leaking. I already had a new one and just left this one in as a sacrificial part to keep the tank plugged at both ends.

Here you can clearly see the hole in the lever part.
IMG_20180903_195725.jpg

I subsequently cleaned out the tank by filling it up with water a couple of times, shaking it for a few minutes and then emptying it, removing some of the stones. I repeated this process four or five times before all the stones and debris were removed. It really took some vigorous and elaborate shaking to get every last stone out of the tank. As a final step, I removed all residual water by spraying large quantities of WD 40 in the tank, making it puddle on the sides of the tank and then absorbing it with some rags. This last step is absolutely necessary to prevent flash rust from immediately forming. Afterwards I filled the tank up to the brim with fresh fuel and the project was finished.

The result:
IMG_20180903_200510.jpg
 

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I've derusted a handful of tanks with apple cider vinegar and it stripped out the rust every time. Key thing I found was to fill it to the brim (no diluting with water), do it over three warm days leaving it out in the sun, and after emptying out and rinsing with soapy water, use a blow dryer to make sure it's good and dry, then fog the tank with WD40 to prevent flash rusting. Also use an compressor to blow out the reserve ports. Try to fill it with gas as soon as you can after that and you're golden. I haven't tried the chain trick but definitely will next time.

Apple cider vinegar is cheap, non toxic and the only drawback is it takes a few days. I've also used it to the the corrosion off a pair of rusty fork stanchions.
 

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I think there are som "kits" for doing this, and they contain som kind of paint (sealer?) to use after the tank gets cleaned. To not leave the metal inside the tank unprotected, and prevent more rust to come.
 
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