Here's a repost of a method I used quite successfully. There's no dangerous chemicals to spill; a consideration for me since I: a) live on a lake, and b) drink from a well. There's also no ball bearings or nuts to recover afterwards. You need a 10 amp battery charger, duct tape, a box of salt, water, a funnel, a piece of steel rod, electrical tape, methyl hydrate and light oil. I adapted it from a technique used by antique outboard restorers:
Prep the tank by removing the petcock and gauge sender, clean the paint work around the holes well, and cover the holes with duct tape. Stuff rags inside the hump to re-inforce the duct tape over the sender holes.
Pre-mix the saline electrolyte using 750 grams (1.5 lbs) of table salt to 5 gallons of water, and fill the tank. Tape up the end of a steel rod (I was told re-bar, but used all-thread ready rod which worked just fine) so it cannot short out against the tank bottom. Connect the negative lead of a 10 amp battery charger to bare metal on the tank (I used the gas cap mounting screw so as not to damage any paint). Place a wide-mouth plastic (non-conducting) funnel in the fill hole and put the rod in the tank through the funnel. Connect the positive lead to the electrode. It's best to use a charger with an ammeter. Mine just has a silly voltmeter gauge, so I connected the positive lead through a separate ammeter. Add salt as required to bring the current up to 10 amps. (The best I got was 9.4 amps, more salt didn't help after that). Check every 2 minutes, and stop when the rust is gone. (It got so murky, I couldn't see the hump after a while unless I sloshed out some of the water and tipped the tank. I don't know how they do this with a marine tank and still check it ;-)
Total time for me on this tank was about 10 minutes.
I was amazed at the results. Where there was heavy rusting, clean shiny metal magically appeared. I used a piece of cad-plated ready-rod for the electrode, and it turned black. The saline turned a weird shade of green, with chunks of rust floating around. I was concerned that the left half of the tank wouldn't get equal treatment, and I made a point of draining off some water and sloshing it around to evenly distribute the salt, and an examination through the fuel gauge hole confirms it worked on that side too.
None the less, I think next time I would start off with a saline solution instead of plain water, now that I know how much salt to use (just about 1 kg box to get 9.4 amps). I also blew a fuse in my ammeter when I inadvertently touched the rod against the hump in the middle of the tank trying to stir the salt with the electrode. I guess with a motorcycle tank, not only the bottom of the rod should be insulated with electrical tape, but part way up where it might contact the hump as well.
I flushed everything out thoroughly with water after, followed with methyl hydrate. Even so, I could see a faint patina of rust starting to form again within 15 minutes. I quickly coated the inside with oil, and that took care of that.
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