So, I have had my k100 for a few weeks now, but it was only rideable for the first few days because there are a few things I needed to fix. When I was removing the shock to send it in to get rebuilt, I noticed some liquid on the left side of the engine, right underneath the fuel injector bar. It took me awhile to find out where it was coming from but eventually I found the source, a weeping hole in the fuel tank, previously repaired with jb weld or something similar. 

Now here is where the story gets embarrassing. I had just filled up the fuel tank earlier that day and I apparently wasn't thinking clearly, because I thought it best to investigate this tiny hole with my hands while the tank was on the bike and full of fuel. The resulting chaos probably provided an interesting scene for the neighbors as I spent the next 20 min or so alternatively trying to plug the hole or capture the gas pouring out of the 1/8th in hole that was left after the jb weld failed. 

One good thing that came from the escaping gas was that it showed me that the valve cover was painted black with pretty crappy paint that came off easily when exposed to gas. This may not sound good to you but I was wanting to take the valve cover and crank case cover back to al, which I believe is stock, and this leak showed me that removing this paint will likely be pretty easy. On the flip side it also revealed that the original color of the bike was blue and that the black paint on the bike is also of a pretty low quality. 

Alright back to the tank, and it's repair. Other than the hole, the tank was not in bad shape, there was a bit of corrosion and a bit of lacquering but all in all it wasn't bad. I decided to go with the Caswell treatment instead of others because it is resistant to ethanol and it seemed highly regarded. I followed all of their instructions for prep which took me alot longer than I had imagined.

First I filled the tank with drywall screws and acetone and shook it up. This worked ok but after much shaking and tired arms the inside was still pretty grimy. so i took all of the screws out and used a scotch brite pad and acitone on all of the surfaces that I could reach inside the tank. This worked really well, I did the drywall screws and shaking again before moving on to the next step.

Next was lye (draino) and water for 30 sec, then rinsing with water, then vinegar and water to neutralize the lye, and lastly a final water rinse. 

Finally I could apply the product. Caswell is a two part epoxy, so I mixed the two parts as well as I could and poured it in. (I had earlier taped off everything that needed taping) The epoxy went in very thick and initially I was sure that there was not enough, or that it was too thick, because the epoxy moved very slowly around the tank as I was trying to get it to coat everything, it ended up being ok but it took a full 20 min of moving the tank around to get the epoxy to run over all of the surfaces. 

The instructions sat to to pour out the excess when done coating and i tried but it seems like the kit I bought was the exactly correct amount because it did not want to come out once I was done coating. 

Here is a picture of the finished product, I tried prioritize the coating of the walls not the interior tubes and mount for the fuel pump so those are a little exposed but I think that that should be alright. Let me know what ya'll think.

-Gene

Interesting read. Thanks Gene. What was the ambient temperature when you mixed that epoxy? Higher temps will set the epoxy faster. That's maybe why it was thick??  Looks like a job well done. Let us know how it goes in the future. Cheers

The ambient temperature was around 70 deg which is right on according to the directions. Looking back on the process I think the thing that may have been messed up was my expectation. I expected it to be thinner and freaked out a little inside when it was thicker than I expected.

Gene,
  I used the Caswell kit on one of my bikes and had the same experience. Almost nothing came out at the end. That was six years ago and the tank hasn't leaked a drop of fuel since (I wish the same was true for the rest of the bike ), so don't worry. You probably got it just right.
  -Jon