So, you’ve just found yourself a great project bike! The good news is, the tank isn’t a total rusted-out nightmare full of holes, dents, and shoddy patch jobs, but the bad news is, there is some rust inside. It’s a bike that’s been sitting for a while, so although you wish that wasn’t the case, you’re not exactly surprised at what you’ve found. What do you do?
That’s exactly the type of situation that YouTube channel Brick House Builds covers with this very handy how-to video. In it, BJ of BHB walks us through the process of using electrolysis to safely remove the rust from the inside of a Honda CB750 fuel tank.
The required parts list is simple. He uses a bike battery (a car battery would work, too), clamps, a piece of steel round bar as a sacrificial anode, a modified plastic spray paint can lid to keep the anode in position without letting it touch the metal of the tank, some hot water, and some sodium carbonate. The most important part, though, is simply time. (You'll also probably want a battery charger to keep your selected battery charged throughout the process.)
Electrolysis is effective, but you won’t start to see serious results for at least 24 hours—and maybe even longer. Above all, you need to be patient. Since the amount of rust involved will vary by individual piece, you should also be aware that you may need to use more than one round of sodium carbonate solution to get all the rust relocated safely away from your steel part.
Luckily, as BJ mentions, sodium carbonate—which is sold for swimming pools as “pH increaser”—is pretty inexpensive. Also, you’ll only need about a cup of sodium carbonate for each five-gallon bucket of hot water, if you’re following BJ’s recipe. If you already have a spare bike or car battery sitting around, as well as some clamps, probably the most expensive part of this proposition is your time. If you’re spending it doing other things (like working on other parts of your project), then that’s probably just fine.
Afterward, you’ll just need to run a bunch of water through the tank to rinse everything out. Once you’re done rinsing, if you’re not planning to immediately put some fuel in the tank, you may want to use something like fogging oil to prevent flash rusting. All in all, it’s a pretty straightforward process, and one you may find yourself using a lot if you decide that you like working on project bikes.