cjoe,
I'm right in the middle of this on my 51. Was pretty rusty inside when I started. First thing I did was flush it out real good with plain old water, till it ran clear. Blew it out and then let it dry. I then put about 40 2" drywall screws and about an 8' length of that cheap flat link chain that they sell for hanging flourescent lights, etc. I would shake it and rattle it around for a while every day for about a week. Then dumped everything out and flushed it with water again. This will get rid of the bigger chunks of rust. Then I took a half gallon of muriatic acid and added it to a half gallon of water inside the tank.(Always add the acid to water and don't breathe the fumes.) Shook and sloshed this mixture around for a couple of hours, drained and rinsed. Followed the same procedure a second time. Then filled up the tank all the way, added a gallon of the acid, and let it sit overnight. The next day I drained and rinsed again, then put in a quart of hot water mixed with a quart Marine Clean which is available from POR-15 dealers. Sloshed that mixture around for 24 hours( no you don't have to do it continuously ), drained and rinsed. I'm at this stage now and you'd never know it was the same tank. Nice clean metal inside. I plan on giving it one more overnight acid soak tonight, and then another 24 soak in the 1 to 1 Marine Clean solution. After that I'll treat it with POR 15's Metal Ready, which is an acid solution that will etch and treat the bare metal like Fumblin says. I've also got a quart of POR-15's tank sealer, but I haven't decided yet whether I'll use it, since the tank is solid and very clean now.
The muriatic probably removed the terne plating from the inside so my only concern is now keeping future rust at bay.
The muriatic I purchased at Menards for $3 a gallon, but any hardware store will carry it. The Por products I bought at a swap meet, for about $60 total. Gallon of Marine Clean, quart of Metal Ready, and quart of sealer.
Like Viejitos says, its easier to take it to a radiator shop, but I'm a little stubborn and like to do things myself.
Heres a link to a good site about cleaning a fuel tank:
http://www.ratwell.com/technical/FuelTankRestoration.html Bob
