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Join Date: Mar 2006
Location: Kyle, Texas
I won't rag on the other methods used here, but I like the acid method primarily because it is fast, and the most effective I have seen. Whole process takes about 3 hours. This is from a tank that was like brown mud inside from all the rust, to a bright shiny silver, rust free interior. If I had only shot before and after photos!
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There are different methods used to clean the rust out of a tank. My favorite is using acid. This makes some folks queasy here, but it does a really good job, is cheap, and pretty quick, compared to other methods.
After removing the tank, you need to flush it thoroughly with your garden hose. Really work hard to get everything you can out with water. You have to remove everything, petcock, fuel level sensor, gas cap etc. and tape up every opening with good quality duct tape. Use a handful, maybe 50 or so, small 1/4" steel nuts (not 1/4 inch threads, but small steel nuts that are about 1/4" outside diameter) and dump them into the tank. Carefully, pour about 12 oz of muriatic acid (they sell it at pool supply stores, or Home Depot) into the tank and tape up the gas cap hole and shake the tank vigorously for about 10 minutes. Keep the garden hose handy and rinse off any leakage immediately. (wear old clothes, rubber boots, rubber gloves and eye protection, maybe a smock if you have one) The acid and nuts will eat the rust off the tank inside. You might have to vent the pressure off the tank by lifting the tape around the gas cap hole every once in a while. You'll know when it bulges the tape. Keep rinsing the outer surface immediately if any acid leaks out around the tape. It will be the color of mustard so it's not going to be hard to see. After 10 minutes or so, remove the tape and let the acid drain from the bottom of the tank, into a plastic bucket full of water that you have added about a cup of baking soda. The baking soda will neutralize the acid. The writeup said to pour the acid on a patch of old concrete that you don't mind staining to neutralize it, but I prefer the baking soda method. Then you can pour the mixture in a gravel or dirt area and it will be fine. This acid will etch the metal and your surface will be clean and rust free. Now rinse the tank very well, flushing it with the hose and shake it to make sure you get all the acid rinsed out. You can use a telescoping magnet pick up tool to get all the nuts out of the tank. (make sure the nuts are steel!) Then tape it back up and pour two bottles of naval jelly into the tank. You need to do this immediately after you get the acid flushed out, as rust will start to form immediately on the untreated metal. Shake the tank for a minute or two and then let it set for 30 minutes, then shake it again. Do this for 2 hours. Then rinse the naval jelley out by flushing it with the hose. Once clean, dry off the outside and rig a blow drier at the warm, not hot, setting and let it dry the inside of the tank. The phosphoric acid in naval jelly will treat the metal and make it resistant to rust. My tank has been silver inside since I used this to clean it. (I did this on my KZ1000P when I first bought it, and I sold it 1.5 years later and the tank was still nice and silver inside, no sign of rust!)
It's messy to do, but it works very well, and doesn't leave a film inside the tank that you might have problems with later. This method might seem a little radical, and some of you might prefer not to mess with the acid, but it works very well.