Join Date: Aug 2009
Location: San Jacinto, Ca
Copied from another site
Motorcycle Gas Tank Rust Removal Technique
1. Get a Kreem Fuel Tank Liner. Available at Dennis Kirk, part #3160, $14. You can also order the complete tank prep kit for $26 if desired, but in compliance with CBMMA, I took an even cheaper approach. The kit basically has three parts: part A (acid), part B (MEK), and part C (the liner).
2. Empty the fuel from the tank if it is not already. Remove the petcocks and fuel filters from the tank.
3. Block the openings for the petcocks. I fashioned two stops using a couple layers of heavy corrugated cardboard wrapped in duct tape, cut to fit and held in place with the petcock screws. I sealed them on with cheap silicone sealant (which comes off fairly easily afterward). A wood or metal stop might be more appropriate, but this worked fine for me.
4. If the tank sat with gas in it that evaporated (like mine), clean the tank with a little acetone (or MEK) to remove the worst of the varnish left by the gas.
5. If the tank has large chunks of rust, you may want to add BBs (or chain, etc.) and rotate the tank for a bit to remove the worst of it. Mine didn't need this since it had only small flakes of loose, powdery rust.
6. Mix up a 10% solution of muriatic acid. This is available from most hardware stores as a 20% solution, so just dilute it with an equal amount of water (always add acid to water, not the other way around). I mixed half a gallon of acid with half a gallon of water for my tank. Pour the solution into the tank, taking care not to get any on the paint, unless you plan on repainting. This goes for the acetone or MEK as well.
7. Set the tank on one side for about ten minutes. Then switch to another side for ten minutes... etc. so all the sides get cleaned. I let the whole tank sit with the acid solution for a total of about an hour and a half I think. Look through the gas cap carefully with a flashlight to check the progress periodically. The tank should look brand new inside when done.
8. When all the rust is gone, dump the solution and rinse the tank very thoroughly with water. Immediately clean all inside surfaces with a little acetone or MEK and empty out as much as you can when clean. Any small amount leftover will only slightly thin the liner (which contains MEK anyway). Do not let all the MEK evaporate completely because oxidation will begin right away. Immediately proceed with the next step.
9. Pour in liner according to its directions and follow remainder of directions from liner.
10. The liner may end up a bit lumpy looking, but this is okay, the tank at least sealed and protected. I would recommend doing the whole process outside (for fumes) with a hose handy for diluting any spilled acid. The acid bath may open leaks in the tank if the rust was very severe, so be prepared. If not too large, these can be patched with epoxy filler or by soldering. I was fortunate enough not to have any leaks. I should also mention you may want to check the gasket on your gas cap before you begin any of the process. Mine was so hardened it did not seal properly, but I found a thin rubber gasket at a plumbing supply store for $1.50 which I fit on top of the old gasket for a perfect seal.