I have a 1930 model A with a rusty gas tank. How
The gas tank sealer works fine so long as it is done properly. You have to remove your hood, cowl band, speedo cable & all wiring that run from the dash panel down through the tunnel under the gas tank. Remove the fuel line and fuel valve under the gas tank. Remove the row of pan headed screws that attach the gas tank to the firewall. On the back side of the tank, there is another row of bolts that must be removed. There are clips that pinch the gas tank to the cowl panel on each side, remove all of them. Now the tank can be pulled out the top.
Once the tank is removed, you will have to remove the gas gauge and the filler screen that is in the filler opening. Before going any further, I would recommend that you mask off and cover the painted parts of your gas tank. If the sealer gets in contact with the paint, it can harm the finish. This will also protect the paint from scratches or chips when you are working with it. Put a pipe plug in the fuel valve hole and tape off the gas gauge hole with some duct tape.
You will want to get as much of the rust and scale out of the tank before you seal it. I have found that a steam cleaner with some rocks will do a good job. I put a couple of gallons of hot water in the tank with about 5 good size stones that will fit in the filler opening. Put the gas cap back in place and slosh the tank around in various positions. The rocks will break up any rust that is stuck to the tank. Once you have sloshed the tank around, drain the water out. Repeat this process until the water comes out clean. Be sure to remove the same number of stones from the tank that you put in.
Let the tank set in the sun and dry for at least 48 hours before sealing the tank. Do not force dry the tank with a hair dryer or heat gun as this will normally put moisture in the tank. Gas tank sealer will not stick to a moist surface.
Once you are sure the tank is completely dry, pour the tank sealer in and put some duct tape on the filler neck. Do not put the gas cap in place as a plug becuase the tank sealer will get into the vent holes and plug them up. Slosh the tank all around and drain out the excess sealer. Set the tank so that the gas valve hole is the lowest place to be sure to get all of the excess sealer out. You will get quite a bit out on this first coating. Let the tank dry for at least 24 hours and repeat the sealing process again. This second coat will give a good coating. After you have drained most of the excess sealer out on the second coating, set the tank so that the steering column support is the lowest position. This will let the excess sealer settle where the column support is riveted to the tank which is a common leak area. Allow to dry for about 24 hours before installing back in the car. Take a sharp O-ring pick and clean the sealer from the threads of the gas valve hole. Reinstall everything and you are good to go.