Fuel tank cleaning help.

[MiCarl]

I used to own a motorcycle repair shop and I did quite a few rusty fuel tanks.

Start by removing the sediment bowl. It's pot metal and the acid you use later will turn it to swiss cheese. Put a pipe plug where it came off the tank.

Then some warm soapy water (probably a quart in a cub tank) and a hand full of 1/4 " nuts or similar. Agitate and rotate the tank (will be tough with a cub tank!). Dump and rinse well. This will get out any big flakes of rust and remove all fuel residue.

Next I'd use phosphoric acid to remove surface rust, although I suppose muriatic works too. The nice thing about phosphoric acid is it's pretty mild, they use it to give Coke it's tart taste. You can get a gallon jug in the paint section at Home depot. For a cub I'd use a full gallon and completely top off the tank with warm water. Pin holes can be closed off with plumbers putty.

Now you wait. As the rust dissolves there will be bubbles that will foam out the neck of the tank. It won't hurt the paint but it shouldn't be sitting anywhere the acid spilling will cause a problem. Once it stops foaming look through the liquid with a flash light and make sure the tank is clean. If it's not the acid depleted before it was finished and you'll need to do it again.

Then rinse well. As stated after an acid treatment it will flash rust in a hurry. I usually filled the tank full with clean water to keep air away although baking soda to neutralize the acid isn't a bad idea. Once I was confident the acid was away I'd dry with a heat gun.

Grease the pipe plug in the fuel outlet and reinstall it. It's also not a bad idea to smear a thin layer of grease on the filler neck where the fuel cap contacts it.

I used Caswell tank sealant. It's a 2 part epoxy that will fill pin holes and presumably the rest of the tank could rust away and it'd still be fuel tight. They say some surface rust isn't a problem, that it actually helps the epoxy adhere. On motorcycles I was careful not to have any surface rust because the thanks are shallow and my customer would be able to see it through the clear epoxy. Anyhow you mix the epoxy, pour in the tank and agitate every which way to get complete coverage. Dump any excess. I always masked carefully around the fuel filler because the epoxy will stick to the paint very well. As the epoxy gets rubbery (watch the excess you dumped) you can use a box cutter to clean up just inside the filler neck so there won't be build up that interferes with the cap. Pull the pipe plug and open the outlet while the epoxy is still not fully set.

Once the epoxy is fully cured put everything back together and you're in business.

The only tank liner I ever saw that failed is Kreem. If the prep isn't done perfectly it'll start to peel off and make a mess that's impossible to clean up. Also the chemicals in it will wash paint right off the tank.

[Gary Dotson]

That sounds like a very thorough process, Carl and a good explanation of it, as well. I’m not familiar with Caswell tank liner, where do you get it and what size kit is available? I also like the use of phosphoric acid, rather Muratic acid, a much more gentle, gradual rust desolving process. I’ve used it in the past for soaking / derusting small components. No instant gratification but excellent results.