After spending the last 3 weeks to clean a Goldwing gas tank, I thought this would be a great topic for this week. In messing with any motorcycle that has been sitting, you will encounter a one of the following scenarios;
1-Rust on the inside
2-Old Gas still in the tank
3-A bad tank seal job
4-Pinholes in the tank
I hope for just some rust on the inside of the tank if it's not clean. This is quite easy to treat. I use metal rescue in the tank for 2 days. I've had great success with this and it looks brand new right after it.
If there is old gas, depending on how long it has been sitting, you may be able to dump it and give a quick rinse with acetone. I had one that had gas that sat for about 20 years and was now some type of gel on the bottom of the tank. I have seen some that leave a hardline where the gas line is and have a type of scale on the tank above the fuel level. In both of these scenarios, I used apple cider vinegar to clean the tank. When you dump the vinegar, odds are it will flash rust quickly. Once this happens, I do a treatment with metal rescue and then a quick rinse with acetone.
If the tank had a bad seal job, get ready to have some fun. I have had some come loose in 3 days in vinegar with some bolts to slosh around and another that took 3 weeks and also had rounds with air craft stripper before the vinegar. Make sure you get all of it out. Then follow the same steps are before.
If the tank has been welded to repair pin holes, or from repair work, then I always use a good quality gas tank. I've tried POR15 and Caswell. That being said I only use Caswell now. Follow the instructions to a t and it will work great!
Keep in my mind this is my approach, there are some many methods you can use. I hope this helps some people.