I am in the process of rebuilding my CB750 K1 and I have started working on the gas tank. Looking in the tank, I could see easily see rust, especially at the bottom, so I decided to remove the rust before I continued with the tank. I tried "The Works" trick and that did nothing, then I tried Evaporust and that also did nothing. I also tried a product called Marine Clean that also did nothing. Then I realized that the previous owner had put a clear liner in the tank over the rust. Why they did not remove the rust before putting the in the liner is beyond me -- the tank was not rusted through in any manner (it's actually in nice shape).
I searched online for the best method to do this and I saw different suggestions, mainly putting in some liquid that will dissolve the liner (Acetone was recommended, as was any lacquer thinner that contained Methyl Chloride). I tried all of those (with soak time from several hours to several days and I was starting to weaken the liner (and it was coming off in some places), but it was a slow process. I then saw a post that said a radiator shop might be able to boil the old liner out so I called a radiator shop and spoke with them about doing this. They said they could not boil out the liner -- it would not work. They said the only way to get the liner out was to burn it out by sending it to a burn oven service. They would heat the tank to 500 - 600 degrees and it would burn off the liner. The cost was $75 for that service, but it would take 7 - 10 days to get my tank back. He also said if I could fit the tank in my oven, I could put it on self clean and that would get hot enough to burn out the liner, but caustic smells would go through my house. That wasn't an option because my wife would kill me.
WARNING: THE FOLLOWING WILL RUIN YOUR PAINT! (I was repainting, so no worries for me)
So, the oven was out, but I do have a very large gas grill that would fit my tank and can hit 500+ degrees. I put the tank on the grill (cap was removed, petcock was out -- it was just the bare metal tank), put it up on high, and it hit about 520 degrees. I let it stay on the grill for about 25 minutes at that temperature. I then shut off the grill and let the tank cool. The liner had burned off and turned to charcoal powder. I hosed out my tank and dumped the remains of the liner into a bucket and there were just a few places where the burnt liner was stuck to the sides. It was a 100% improvement.
Since the tank immediately flash rusted since I had used water to rinse out the liner, I went ahead with The Works de-rusting method. I filled the tank to the top with a combination of water and 2 bottles of The Works liquid toilet bowl cleaner and let is sit for about 30 minutes. This took care of the rust and the few spots where the burnt liner was on the tank. I then dumped that into a large bucket and rinsed the tank twice with a water and baking soda combination to stop the acid in The Works. I then poured in two containers of rubbing alcohol to displace the water (you can use Acetone as well, but I was out and rubbing alcohol works and is less caustic). I dumped out the rubbing alcohol and used and air compressor and heat gun to dry the tank and prevent rust.
Below is a before and after picture. The before picture does not show how bad it was. It shows the rust, but the liner was thickest on the bottom of the tank so that's where I had the most trouble getting it out -- I don't have a picture of that. Ignore the rust on the top of the tank -- that I will remove before I paint.
Anyway, I hope this helps somebody with the same problem I had.
