So I got 4 gallons of Metal Rescue from Home Depot and let it sit in the rusty tank for 24 hours. I was concerned about the temperature dropping below 68F; the minimum recommended temperature for the product to work, so I picked up a heating mat from Walmart and placed that over the top of the tank, figuring the most surface area contact would work best, then wrapped the tank in some towels and my welding jacket to insulate it and keep the heat in. I also picked up a laser thermometer so I could accurately measure the temperature: if I'm spending $100 on Metal Rescue, another $40 for a thermometer and $30 for a heating pad seems cheap insurance for getting it right. (With the heating pad on full blast the tank was 94F at the top and 82F at the bottom.)
And the results after 24 hours? Pretty amazing. I used some 1/4" tubing to syphon off the Metal Rescue, first into a bucket so I could see the color, then directly into the old containers. It has a shelf life of about a year, so I can use it again. Once it turns completely black then it's finished and you can just pour it down the drain. Apparently it's that environmentally friendly.
Towards the bottom of the tank things started to get a bit murky so I switched back to the bucket, and you can see that all the dissolved iron oxide had settled to the bottom and the remainder of the Metal Rescue is black. Using a strong flashlight I inspected the tank; the majority of the rust is gone, but there is still some hardcore traces on the bottom. I am figuring that the Metal Rescue got fully saturated down there and couldn't do such a good job as up top, especially where it was warmer. So I've put back in the remaining clean product and placed the heating pad under the tank, again covering it with towels/jacket for insulation. Another 24 hours should clean the tank completely.
The OEM petcock still functions but is a bit stiff to operate. It also is vacuum assisted, and since the Mikunis have no vacuum port to hook into I am going to replace the whole petcock with a simple one I found at Dime City Cycles. Of course this will mean having to switch on/off the fuel every time you ride, but a small sacrifice to make. Besides, I think there is something romantic about pre-motorcycling checks on classic bikes. Time to slow down a bit and appreciate the fine motorcycle you are about to throw your leg over and trust implicitly.
Current hours on build: 184.0