I have a 1980 GS450L that had a nice dent on the right side of the tank about the size of a grapefruit. After much banging and wrenching and so on, I gave up trying to get it out. It was just not worth it. I went to a local motorcycle junker and got an identical tank in a different color that was dent free, but had some rust.
I used paint stripper and sandpaper to remove most of the rust, and then dipped the tank in an electrolysis bath to remove the remainder. This works remarkably well, especially if you orient the parts so that the heaviest rust is near to the electrodes.
After that, I discovered that some of the rust had made it's way through one of the welds that a PO had made to repair some earlier damage, so I patched it with some brazing solder and a pipe torch. After filing it smooth, I sealed the tank with some Caswell tank sealant, which is like maple syrup it's so thick. I chose to thin it a bit with some lacquer thinner just to make sure i got all the surfaces. I made sure to plug up the mounting holes for the petcock with tape so that they wouldn't be obstructed when I put it back together. The cured product is nice because it's hard but flexible. I had worried initially that it would be brittle and might crack if i dented the tank some time later but I'm confident it will hold up well.
Right now I'm in the process of fairing out the rusty spots and dimples, and prepping for paint. I've chosen to put some Underbody rust preventative paint on the bottom of the tank. It's not visible, and it's subjected to the most road dirt and debris. It's also where I found most of the rust (up by the front mounting grommets). Since I'm painting the tank black, it won't even be that obvious, and will resist wear at the points where the tank rubs the frame/wiring harnesses/brackets/etc.
I've also bought some high build primer and an epoxy paint for the color. I'm hoping it will hold up well to any fuel spilled and any heat it may catch as part of being above the motor. See the link below for a few pictures of the process. Hopefully I haven't missed anything.
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