My son just purchased a motorcycle that has not been run in a few years. The inside of the tank looks really bad but doesn't show any signs of rust that we can see. Any ideas on the best way to clean it out?

Radiator shop for a cookout. Of course, this action will probably remove the paint.

Eastman does sell, as others I'm sure, an epoxy resin you pour inside and roll the tank around to cover up the inside surface/rust.

Look inside with a small mirror, and if there is no major rust just flush out the dirt with some gasoline. Remove the valve from the tank and use the hole to drain the tank, Its hard to drain most tanks through the filler hole since they are protruding. If the valve has a small filter on it, remove this before reinstalling because it gets clogged real quick. Put one of those clear plastic inline fuel filters in the line to the carbs to catch any left debis. These filters are cheap and can be much easier replaced than the one on the valve.
If the bike hasn't been run for a long time you might want to check the valve on the tank, some of them start to leak after a long time of no use. So don't fill the fuel tank to the top for the first time, just put in half a gallon to check it out. You might want to remove the floatbowls too and check those for dirt.

Dutch you could use the same procedure as for coating it with POR ie "Marine Clean" to disolve all the varnish and then "Metal Ready", which is phosphoric acid, to convert any or all of the rust. But then just skip the coating part since it's already in good shape. But...I hang out on a few old motorcycle sites and believe it or not the veterans will tell you the cheapest way is to rinse it good with lacquer thinner to get rid of the varnish and then....you ready ?? a case of Coke Classic, which also contains phosphoric acid. Just plug all the holes, fill it to the top, and let it set for a couple days. They swear it really does work:thumbsup:

I bought the POR kit to restore an old Ford truck gas tank. It was really rusty inside. It's a three step process. Degreaser, metal cleaner, then an epoxy coating. 5 years later and all is holding up well.

I had read the stuff about coke and phosphoric acid. I don't think you can every really get the scale and little rust particles off. So I would use the epoxy to seal it in.

I put a set of after market fat bob tanks on an old Harley a number of years ago and since the welds are prone to leaking, I bought a kit that had a metal etching liquid and epoxy paint included. I let the etching liquid sit for a couple of hours, dumped it out, then put in the paint, rolling the tanks around to coat all surfaces, dump out the remainder of the paint and let it dry for a few days. Rode the bike for 5 or 6 yrs.

If $$$ is an issue, just rinse with laquer thinner and give it a try. What are you gonna lose? Jim