The Columbia tank had a good coating of rust inside, and I decided to do the acid cleaning thing. Here's what I did, and how it went:
Removed the petcock, and rigged up a tube for the tank fitting. Clamped off the tube with a hemostat. My first error was neglecting to soak the tank with paint thinner first, more on this later. I filled up the tank with full-strength muriatic acid ($1.79 a gallon) and let it work.
My second error was slightly overfilling the tank with acid. As it worked, the gas produced caused the tank to burp occasionally, which was slopping drops of acid onto the frame and motor. Used a turkey baster to lower the level, and it was fine after that.
The acid worked hard for a couple hours, and then slowed down. Drained it off and what I could see of the tank was totally spotless - nice. Flushed with water from the hose for about 10 minutes, and drained it all off again.
When you clean a tank with muriatic acid, you really should finish off with a phosphoric soak, to condition the metal and prevent almost immediate re-rusting. I'd tried about 3 places, and there was no phosphoric acid to be found. So, I did the next best thing - I dumped 3 liters or so of coca-cola into the tank, and let it work. And work it did - fizzed and sizzled away in there almost as hard as the muriatic did.. coke is nasty stuff.
Half an hour later, I drained the coke, drank it (kidding), and flushed again with water.
Now came the denatured alcohol to dry up any water. Filled the tank and let it sit for about 15 minutes, then drained and saved the alcohol. Here's when I discovered my first mistake - the alcohol came out yellow. There was dried up varnish in the bottom of the tank, and the alcohol was disolving it. I should have soaked it with thinner first. Put some more alcohol back in, and let it soak for an hour. Drained this off (it was yellow-brown).
Reinstalled the petcock, and filled with fuel. Fuel took a slight yellow tint from the remaing varnish in the tank. Hopefully it will burn OK, and I don't have to dump it - that synthetic oil is too pricey to waste.
One thing I noticed, is that the columbia has a pretty small tank capacity. Don't think I got much more than 0.7 or 0.8 gallons of anything in or out of that tank.
Hope some of this is useful info. Comments always welcome.