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A thin film of rust isn't unusual on older bikes. You can wash it out with phosphoric acid to remove it. You can find it at Home Depot, sold as etching concrete cleaner. A slow, high-tech way is to use sodium carbonate (Ph Up at Home Depot in the swimming pool/spa supplies) and a battery charger (6 amp minimum). This will leave a zinc oxide coating in the tank that will prohibit future rusting. Kreme and Por 15 should only be used on badly rusted tanks that cannot be economically replaced. If the tank is not prepared properly, they will begin to flake off in a couple of years. Almost as many tanks have been ruined using these as have been saved. The new formulation of Kreem is now removable, which may be an improvement. Por 15 has a slightly better reputation, but I've never used it.
From: Curby Keith
Subject: Re: Re: Rust in your tank?
As long as you have a good fuel filter and change it regularly, it shouldn't cause any problems. Without a filter, it will eventually cause carburetor problems. Some of my bikes had (have)it (KZ440, EN500, GS500, , Dream 305, Magna), some don't (VN500, GL1200, GL1000, CB175, KZ305). I had some pictures of the process, but haven't been able to locate them yet. Here's the text of an article from the VJMC magazine on rust removal:

Supplies: rusty tank, medium sized steel or zinc plated bolt, piece of 10-14 gauge stranded copper wire, battery charger with 6 amp output (higher is faster), sodium carbonate (pool/spa chemical ph+), and water

Remove the sender if the tank has one; if not, shake out any loose bits of rust and vent the gas vapors from the tank by leaving it open for at least a day. Mix 1/4 cup of sodium carbonate solution for every two gallons of water. Stir it up well and pour the brew in the tank (you might want to make a little extra, as some usually spills).

Clip the battery charger ground lead to the tank body in a clean area. Strip both ends of the stranded wire, one end 1/2-inch and the other end about 2 inches. Split the strands on the long end, and securely wrap the wire around the bolt. I use a 5/16-inch diameter bolt about 2 inches long, either steel or zinc plated. Immerse the bolt in the solution and attach the wire lead to the positive terminal of the charger. Set the charger output on the 10-amp setting.

The mix will start to bubble a bit, and rust and gunk will form around the bolt and upper tank opening. Rinse the bolt off every few hours, carefully pouring additional sodium carbonate/water solution into the tank opening to flush out the gunk there as well. Keep the tank as full as possible.

After a day or two, as the process slows down, the amount of gunk generated decreases. Most of the bolt will disappear as well, so keep an eye on the copper wire holding it; the bolt might need to be replaced. When it looks as if the process has finished, rinse the tank with clear water. Slosh it around and even use a handful of 1/4-inch nuts on a string to scrub out any loose residue. Rinse again and dry with warm air. A vacuum cleaner outlet hose or hair dryer can be used to supply drying air.

If I'm not completely certain the tank is dry, I'll use a quart of denatured alcohol in the first tank of gas to remove any moisture that remains. This process only eats up rust, not steel. It converts rust to black oxide, which is a more inert material. Unlike acid etching, the surface left behind is rust-resistant. Acid etched steel left bare is much more likely to re-rust. This process works well, the chemicals required are safe if properly handled, the tank shouldn't need to be coated afterwards (unless it's leaking), and the time required is minimal.
 
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