Lining a tank
Heres the procedure I've used in the past with good results.... Get a gallon of Muriatic acid from your local hardware store. It is commonly used in swimming pool applications and should be readily available. Then take your tank and empty it of all of the liquid currently in it. Next, using a garden hose or power washer, spray out the inside of the tank to loosen any heavy scale and deposits, and then drain the water from the tank. Block off one of the openings in the tank and pour the entire gallon of muriatic acid into the other opening and seal that opening too. Muriatic acid is not caustic like other forms of acid and is water soluble, but remember it is still acid so be careful in handling it. With both holes sealed, roll the tank around swishing the acid all over, rolling the tank over, up on end and side to side. Allow the acid time to do it's magic, so let it sit on the bottom of the tank for a while, then swish it around again. Do this for an hour or so, moving and sloshing every 10-15 minutes. When you are satisfied that the tank may be clean, open one of the holes and completely drain the tank and dry the inside of the tank with either a hair dryer, leaf blower or some form of rushing air that is not compressed, unless you plan to seal it right away. The muriatic will "etch" the inside of the tank, meaning that it will dry to almost a chalky appearance. This etching will allow the tank sealer to bond with the metal, making for the best operating conditions for the sealer to work. Then, just follow the directions on the tank sealer and allow the sealer to dry completely before adding any fuel.
Hope this helps, Geof