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Gas Tank Restoration

1995 BMW R100RT Classic

Who was the knucklehead at BMW that decided an internal flap on the gas tank filler hole was a great idea? My 1995 R100RT Classic had the dreaded flap. Every time I put gas in the tank, I would get some splashback because you couldn’t angle the gas pump nozzle off to one side of the center seam of the tank. It also prevented me from being able to look inside the tank to see how much gas was in it. After a 2 day trip up to Milwaukee and Madison with a friend this spring and experiencing the frustrations of this tank flap, I decided it was time to remove it.

With the gas tank off the bike (fully drained and washed out), I took a stout screwdriver and pushed the flap down and to the side as much as possible. This allowed me to grab the edge of the flap with my long bent needle nose pliers and twist the heck out of it till it came loose. I didn’t worry too much about pieces dropping in the tank as I would be removing the petcocks anyway, giving the inside a good cleaning.

Now I needed to cut off the lower section of the tank filler tube that the flap was attached to. The tube has 5 raised threads in the top half for the gas cap. Just below that is a raised collar. I removed all of the filler tube below that collar. To do this, I used my Dremel with their cutoff wheel #420. I wasn’t able to get much of an angle working in such a small space, but eating through 6-8 wheels, I was able to get most of it out in one piece.

Finally able to see what the inside of the tank looked like, I was shocked. Obviously, the previous owner(s) had not properly taken care of this gas tank. I got on Amazon and ordered a wireless borescope so I could closely examine the inside of the tank. It was not pretty!

Realizing I had a tank coating that was peeling as well as a lot of rust, I turned to various articles and forums to find out what product people had used and what their outcome was. I decided to use POR-15 Cycle Tank Repair it. There were a lot of positive reviews for the product. The kit comes with three essential products; Metal Prep (basically a rust remover), Cleaner/Degreaser, and Sealer. According to the instructions, Metal Prep is referred to as “Marine Clean” and Cleaner/Degreaser is referred to as “Metal Ready”. Thumbs down on their instructions for not using the same wording found on the two bottles.

I started this process by removing the two petcocks, along with the brass filter screens. To my surprise, there was a partially disintegrated screen laying in the bottom of the tank. The instructions warn about protecting painted surfaces. I ordered a set of rubber stoppers to use for plugging the two petcock holes, and the large filler hole on top.

I used blue masking tape to covered the entire tank. Then I used an old towel and wrapped it around the tank and secured it with duct tape.

The rubber stoppers were pushed into the petcock openings I then took a piece of duct tape and laid it over the stopper. I secured the tape with a piece of wire wrapped around the threads. I did not want these stoppers to accidentally get knocked out with acid in the tank.

As per the instructions, I mixed the Metal Prep (“Marine Clean”) with a quart of warm water and poured all of this into the tank. It says “this product cleans varnish and rust out of the tank.” My experience, it did next to nothing. After agitating it in the tank for nearly an hour, very little rust/old sealer came out of the tank when I rinsed it out. I decided I needed to start looking for a different rust remover before continuing with the rest of this kit. I tried several different chemical strippers (2 Minute Remover Advanced, Jasco Premium Paint & Epoxy Remover) to remove the old red sealer in the tank. I might as well have tried water. None of the strippers did a thing. After searching YouTube for other rust removal methods (vinegar, electrolysis), I decided to try muriatic acid. I bought a gallon at Menards for $4. Not being sure what strength to use, I started with a 50/50 solution. I respected the danger of this chemical by wearing gloves and using it outside, being careful not to breathe the fumes. As soon as I poured in the 2 gallons of 50/50 solution, the reaction was immediate, with a chemical gas cloud spewing from the tank filler hole. After letting it work for about 30 minutes, I put the rubber stopper in the filler hole and turned the tank upside down. Soon after, the solution started coming out the vent hole next to the filler hole. I quickly removed one of the petcock stoppers to make a way for the gas to escape.

After another 30 minutes with the tank upside down, I poured the solution out into a large plastic container. Removing all the stoppers, I washed out the tank with the garden hose. A lot of crud came out. I started up my high-pressure washer and removed a lot more crud.

Using muriatic acid definitely produced results. It was both removing the old tank coating and removing the rust. 

However, there were still some stubborn spots that were not coming clean. The next round, I poured a full gallon of undiluted muriatic acid in the tank making sure to leave one stopper off so gases could escape. I left the acid in for about 30 minutes while flipping the tank several times. Again I used my pressure-washer afterward to remove all the debris. This time, the tank was free of rust and had only a few small spots of the old sealer still stuck to the metal. I figured if it was still holding on after all that, then I didn’t have to worry about it.

Next, I used the Cleaner/Degreaser (“Metal Ready”) according to the instructions, rolling it around in the tank for 30-45 minutes. Afterward, rinsing out the tank with water. At this point, it’s critical that the metal doesn’t flash rust before you put the new coating on. I immediately began drying the tank with a hairdryer. I propped the tank up on a board so the petcock holes were off the bench, allowing the hot air and water to pass through and out the bottom of the tank.

I kept the hairdryer going for at least an hour on high. The whole metal tank was quite warm to the touch. This was also done in my garage at a time when the ambient temperature was about 80°, so that was working in my favor.

I was now ready to pour the sealer in. I wanted to err on the side of having too much sealer rather than just enough, so I had ordered an additional pint of sealer. Hindsight, the half-pint that came with the kit would have been enough. I poured in both containers of sealer, put the top stopper in, and rotated the tank in all directions. After about 5 minutes of constant turning, I removed one of the petcock stoppers and drained out the sealer into a container. Then I removed the other petcock stopper and drained the sealer out of that side of the tank. Now I took the top stopper off and began rotating the tank very slowly to make sure the sealer didn’t pool in any spot till it setup. Within 30 minutes, the sealer was setting and no longer moving. They warn you about cleaning off any spilled sealer. I underestimated just how tenacious this stuff is. It does NOT come off! I had a few spots on my fingers for at least a week. I let it cure for at least 5 days before putting the tank back together on the bike and filling it with gas.

It turned out great! I would definitely use POR-15 again, but I would not purchase their kit. I would buy the sealer and cleaner/degreaser separately and use muriatic acid for the rust removal. Another key component for success is using a high-pressure washer.