Thought I'd better come back and put a disclaimer on this. Do this procedure at your own risk. (Of 7000+ members, there's bound to be someone who "could" do something stupid, like burn all the hair off their wiener dog. I do watch AFV reruns nightly, so I know they are out there...)
This topic comes up periodically as barn stored bikes are discovered and acquired and the new owner shows up here needing help with their rusty tanks, so I’m going to once again, give my opinion with a little scientific (and I use that term loosely
!) research to help calm the fears of using my method. First let me acknowledge the other methods that can be used, and state that I am not knocking those other methods. I’ve not used any other method, so I can’t vouch for them, but others here have and have been successful with them. I chose to use this method over the others because I am somewhat impatient with a process that takes two or three days when I read that it can be done in three to four hours. OK, some might say you could dig a hole with a shovel or use dynamite. Trust me, the acid method is not dynamite! In fact, if you’ve ever swam in anything larger than a kiddie pool, you have swam in various types of diluted chemicals, including muriatic acid.
Muriatic acid is used in just about every swimming pool periodically to help maintain the PH level of the water. A swimming pool PH level is ideally between 7.4 and 7.6 on a PH scale. Anything higher or lower than that scale causes all sorts of issues, including eye irritation and other things, including etching of the pool surfaces, corrosion of the plumbing, cloudy water etc. Often times when the hot sun starts depleting the chlorine in the pool and the added chlorine has to be increased, the PH levels start to creep up and muriatic acid has to be added to bring things back down to normal. This happens especially in pools where chlorine bleach is used instead of the powdered or cake crystals to chlorinate the pool water to fight bacteria etc. Most of the large commercial pools use chlorine bleach, so they also keep a supply of muriatic acid on hand to maintain the balance. If the pool water PH falls below the 7.4 level, the water becomes acidic and soda ash (baking soda) has to be added to neutralize the acidic water and bring the PH back up to normal levels. So, like I mentioned before, we’ve probably all swam in dilutions of these various chemicals and other additives I don’t care to mention or even think about…
So, I’ve said all this to bring us to the discussion of using muriatic acid to clean a gas tank…
Yes, I recommend you at least wear old clothes you don’t care about, or an apron of some sort, long sleeves and rubber gloves when doing the acid cleaning. And absolutely wear eye protection!
We will be using the acid non-diluted so it is pretty powerful at this concentration, and the vapors are nasty, so do it outdoors and if possible keep the wind at your back or use a box fan to help keep from breathing the vapors. Do your preparation for disposing of the chemical FIRST!
This way you have it all set up and ready so you’re not freaking out later—wishing you had. You’ll need two buckets, one to mix your neutralizer in and the other to flush your acid from the tank into. If one bucket is larger than the other, save the larger one to flush the tank in to. Mix one part baking soda into 10 parts water. That’s approximately 1.6 cups baking soda into 1 gallon of water. You don’t need to be overly precise here. If you have a 2 gallon bucket, fill it two thirds full and put 2 cups or so of baking soda in it and mix it up, and you’re good to go. Have your water hose ready to use and keep it nearby so you can rinse any leakage and such when necessary.
I’m not going to go into the specifics here about the etching process, because I’ve pasted the procedure below. This is about the cleanup process. Once you’ve done the shaking of the tank with the acid and nuts in there and are ready to pull off the tape and start rinsing, rinse the acid-rust mixture into the empty bucket (you might need a larger bucket on the rinse side than the bucket you used to mix the baking soda in) until the yellowish orange colored stuff is done coming out of the tank. Shake the tank around with water in it to make sure you get all the areas of the tank rinsed out. If your bucket is getting full, you probably have all but tiny residuals of the acid rinsed out by now, and can let the flushed water run onto the ground, because at this point the dilution is probably less than splashing pool water onto the ground. At this point, you can tape the tank back up and do the first phase of the naval jelly treatment, and once you have shaken it up good and are ready to let it sit for a few minutes, you can come back to the acid for disposal.
Now, pour some of the neutralizer you mixed up in the other bucket, into the acid bucket. It might start to fizz up. This is normal as carbon dioxide is being released as the PH level is rising. Give it a few seconds and pour some more and repeat this process until it doesn’t fizz anymore. If it’s not fizzing, you’re probably in the 7.4 to 7.6 PH level with the mixture. If you have a pool water test kit, you can test it to be sure. If you are in the country and have a septic system, I would probably not put this down the drain because of the possibility that it might kill necessary bacteria in the septic tank. In this case, I would just take the bucket to a dirt or gravel area and pour it while further diluting with the garden hose. It is perfectly safe. If you’re in the city, you can pour it down the drain as you run the faucet to further dilute it.
So… Here’s the full procedure:
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. 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!)
Now tape the holes in the tank 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. This is a good time to go back to the acid bucket and use the mixture of baking soda and water and add it slowly to the acid bucket to neutralize the acid. You’ll know it is neutralized when adding the baking soda/water mix to the acid doesn’t fizz up. You can then dispose of it by one of the earlier mentioned methods. After it’s been 30 minutes with the naval jelly in the tank, go shake it really good again making sure you turn it in all directions while you are shaking it to cover every surface in there. Do this for 2 hours. Then rinse the naval jelly out by flushing it really well 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!)
This method is admittedly a bit messy, but it isn’t nearly as crazy or dangerous as some make it out to be. The one critical time is when you are opening the muriatic acid bottle and pouring 12 oz of it into a measuring cup or pitcher, and then pouring it into the tank. Do this carefully and don’t let it splatter. And this is where it’s kind of handy to have a breeze at your back! Think of it as if it were paint remover. You would be really careful not to get it on the paint so use the same mentality here. This step is where the vapors are strong, and believe me, you don’t want to sniff it. I have, and it takes your breath away! By the way, I’m still alive, and I had a pool for years, and often got to sniff the vapors of muriatic acid when I was mixing it into pool water, so just keep that in mind. (of course some might argue that’s what’s wrong with me
Once you have the acid in the tank and taped off, and are shaking it up like a mad person, the hardest and most dangerous part is over, other than your arms getting tired of shaking the tank. Wouldn’t hurt to have a buddy to share the tank shaking duties with. One could man the garden hose to rinse any leakage while the other shakes…
Anyway, I hope this helps those of you who need to clean the rust, and would like to get er’ done quickly, but are a little freaked out about using acid and such, to ease your mind a little about this process. And for those who don’t mind the extra time it takes to do it, those other methods should work just fine.