Here a liner kit was about $60.00 and I believe had a few steps. clean the flakes, loose stuff out. Some have used small pebbles,marbles, a chain...whatever (I used water as well) will beat the inside and loosen the flakes.Let dry over night. Clean with a chemical in the kit. Dry over night. Prep with another chemical, think it was acetone, and then the liner stuff itself. Rolled the tank around for 1/2 an hour to get as much of the inside covered as possible.
Worked great so far...touch wood...
The tank hole is repairable, my neighbour does those sorts of things but flushes with argon for a while before introducing welding.
Chris

I see reviving an old tank as a three step process.
1. Remove the gunk (old gas, varnish, shellac, whatever-you-call-it).
2. Remove the rust
3. Coat the inside so it doesn't start up all over again.
4. Protect the outside with primer/paint, rust sealant, undercoating or similar products.

I saw a trick of using aquarium gravel. I like the idea because it's cleaner than gravel and more abrasive than marbles. I'm thinking about a soaking with muriatic acid, followed by a degreasing with Dawn detergent, using gravel somewhere in those two steps, and then a liner. The thing that has me holding up right now is what to run thru it prior to the liquid liner to prevent rust from starting back up, or do you just dry it and line it as quick as possible?

Another trick about the de-rusting with gravel.
You put the gravel into the tank and belt it to a small concrete making machine (don't know the english word for this).
Make it roll for an hour or so, or more, as it won't tire your arms, then.
Don't use too small gravel, so that it can fall inside from one side to the other.
Then clean, and redo if necessary.

You can use the same trick with internal coating.
The Restom Company sells kits to coat gas tanks (restom.net and reservoirs).
Not sure they sell in the us.
Have fun !