Tech Tips

How to Fix a Rusty Fuel Tank

Written by  February 28, 2009

I have been repairing motorcycles for several years, and the most notorious leave you stranded problem is: How do I know if my gas tank is rusted or starting to rust? Since most street bike gas tanks, with the exception of newer plastic dirt bike and ATV tanks, are made of metal, what normally happens with age and improper storage is the tank is most likely neglected. When a gas tank is half full, it will be exposed to air and moisture. And if you store your bike inside with the different temperature changes, you can get moisture build up in the bottom of the fuel tank.

Since water is heavier than gasoline, it will be in the bottom of the fuel tank, that’s where the tank is vulnerable to rust. I have seen tanks as new as 2000 that where starting to rust. One of the problems associated with this rust issue is that carburetor idle circuits are very small and the actual pilot jets metering holes are very small. What will happen is that cylinder will starve and foul the spark plug. Usually not a great start to a spring ride. If you have drained your carburetor float bowls and have seen reddish type of liquid or water droplets, rest assured you have a rusty tank! It is time for you to make a decision on whether to pay for repeated carburetor overhauls or fix the problem?

If you have chosen to fix the problem, then you are on the right track for event free riding. Preparation for this requires the tank to be removed and only a couple of hand tools and or sockets. Once the fuel tank has been removed from the motorcycle, designate an area that the tank can rest for a minimum of three days (undisturbed) for drying time. Below are my recommendations for getting the tank ready for acid etching.
I have used Kreem Tank coating with great success, but as with most things in life, the preparation is the most important part. There are four important fine points to consider:
#1 - Tank preparation:
The inside of the tank should be completely free and clear of all loose particulate (rust, dirt and sludge). Even more importantly, it must be totally free of any and all hydrophobic (oily) substances, including any and all traces of gasoline, oil and grease. You will want to remove the fuel valve and if equipped remove the fuel tank sensor. If you leave the fuel sensor in the tank and pressure wash the inside of the tank, you will ruin the sensor. That can be a problem because you are probably wondering how you seal up the sensor or the fuel valve hole. That’s easy... If you have a fuel valve that recesses in the tank by two screws, you can cut a piece of linoleum scrap and make two holes for the bolts, the actual screen filter in the fuel petcock can be removed and reinstalled. Duct tape works well on the fuel sensor, several pieces. You are not looking to hold pressure, only water.
You may not have a pressure washer, which is ok too! A garden hose will give you the water, you can also use pieces of bicycle chain and use this with a little dish soap such as Dawn; this will act as a degreaser as well as let you know when the residue has been removed. Carefully rotating the tank side to side and up and down. You may want to consider that this is a time consuming process and what you are looking for is a rust free tank in the end.
Only now are you ready to phosphate etch the metal in the tank with Kreem kit bottle #1 (phosphoric acid). Although the directions call for a specific quantity of hot water to be added to the quantity of acid provided, it is better to have a FULL tank of etching solution. In the case of the tank, this amounts to adding perhaps 3.75 gallons of HOT water (3.25 for early tanks) to the acid, as opposed to 2.5 gallons, which (I believe) is the quantity specified in the directions. Again, as in the degreasing step, it is vitally important to have the etching solution come in intimate contact with ALL inside surfaces of the tank, including up and around the filler neck. Seal all the tank openings and slosh that bad boy around, but after you're through sloshing, don't forget to leave the filler neck open, I usually take the filler cap completely off and use duct tape, or the hydrogen gas evolved from the chemical process will blow it out of the hole and across the room, probably splattering phosphoric acid solution on your new paint job. Ouch!
Once this etching and hot water solution has been installed, make sure you are not leaking and prepare for at least 24 to 36 hours for complete etching of all surfaces. Tank should be stable where it will not be upset and spill. After the allotted time has passed, you will see bubbles as this is an active acid and what you are looking for is a grayish color in the tank. If you have that color, you can drain the tank, without spilling on the paint; have a garden hose standing by. Once all of the etching solution has been removed into a container, discard the solution and get ready for bottle # 2.
#2 – Water removal:
The small bottle of solvent (Kreem kit bottle #2) is MEK (a close cousin to acetone) and is a very good water remover. After you have thoroughly rinsed the phosphate solution out of the tank, it is time to get things dried up and ready for the polymer coating step. Actually, 'dry' may be a bad word to use here, because the intention is NOT to get the inside of the tank dry from all liquid residues, but rather only to get all WATER residue out. The inside of the tank will still be wet when you are done with this step, but it will be wet with MEK, which is perfectly fine, because that is the same solvent which is used in the polymer to keep it dissolved. Caution: do not dawdle after using the dewatering solvent. IMMEDIATELY go on to the polymer coating step. If you wait too long before you go to the next step, you may 'flash rust' the inside of the tank, which will require starting over. If the inside surface of the tank begins to physically dry out, you're taking too long and are in danger of forming flash rust. You will have to hustle in this step.
#3 - Coating:
Conservatively speaking, there is enough polymer in bottle #3 to do AT LEAST two (probably three) tanks. What this means is that, unless you have two or three tanks prepped all at once, you are going to not use the entire polymer provided in the kit. You must remove the excess polymer from the tank that you are working on. Follow the instructions very carefully here, rolling the tank around for a few minutes, then letting it set on one side, followed by rolling around again and letting it set on another side, repeating until you are secure in the fact that all interior surfaces have been thoroughly coated. After that, you MUST pour out any remaining polymer, or you will have a puddling problem. Even after you pour out the excess, you must continue the process of rolling the tank and then letting it set on alternating sides, while the remainder of the polymer still in the tank forms a non-moving film.
#4 - Drying/curing time:
Probably the one most crucial mistake leading to premature failure of Kreem coatings (after improper degreasing) is insufficient drying or curing time before putting the tank back into service. After you are satisfied that the majority of the polymer in the tank has skinned over, follow this procedure:
Remove all stoppers from all openings, both top and bottom (filler & petcock holes). Turn the tank upside down (into a drain pan) and block it in place so the filler neck is facing vertically straight down. After a few hours, you will no longer be able to smell vapors coming from the tank. It is then safe to move the tank, but DO NOT use the tank yet. Move the tank and drain pan into an area that will not be bumped or disturbed. Cover fuel filler with shop rag or towel. Two days is probably enough, which is why I always do it for about a week just to be sure. It pays to watch the weather and plan to Kreem the tank on a day that is going to be sunny, hot and dry, if at all possible. You will notice that the remaining coating that has dribbled out of the petcock hole and fuel sensor hole has dried and can be broken away from the hole. If you are confident that the coating is completely dry you can reinstall the fuel valve and or fuel sensor. I also recommend installing a new fuel filter in the carburetor line, just to be safe. And that is all there is to it.
If you add the above tips to the (already pretty good) instructions included with the three part Kreem kit, you will end up with a well sealed, carefree fuel tank for many years to come. Good luck!
Please e-mail me with any questions about this tip: I do not want anyone to ruin an expensive gas tank paint job if they are not quite sure of all of the proper steps.
B-safe out there.
Dave Miller