Since I started restoring gasoline engines seven years ago, the same problem comes up - rusted out fuel tanks. It seems to be a common headache among everyone restoring engines. My solution to this has been to fiberglass the bottom and half way up the sides.
There are several reasons why I feel this is the way to repair them. First, and foremost, you can keep them looking original. Second, it is economical and easy to do. Last, and very important, gasoline has no effect on fiberglass, nor can the tank ever rust out again.
Any gasoline tank can be fiberglassed if there is anything at all left of the bottom of it. Materials needed for fiberglassing are fiberglass cloth, resin (with hardener), brush cleaner and a very cheap paint brush. It's most impossible to ever clean a brush out good enough for painting with it again -- therefore get a cheap brush. These materials are available from any marine supply or hardware store. The first step is very important - be sure that you get the tank very clean. I find that using a wire brush on a grinder is the best way of getting these tanks real clean on the outside.
Next, cut the fiberglass to fit the tank. On some tanks you can cut the cloth to cover the bottom and up the sides in one piece by making cuts at the corners and so forth. I always cover the bottom and about half way up the sides. On some irregular and round tanks you may have to cut one piece to cover the bottom and one or more pieces to cover the sides. Do not hesitate to make cuts where needed, but wherever possible overlap these cuts a half inch or so. Now the cloth is cut, set it aside for later use.
The next step is to mix just enough resin for a thin coat on the tank. Follow the mixing directions which will come with the resin, as different companies use different formulas. Remember, mix only what you will be using in fifteen minutes or so, as once the resin and hardener are mixed it will set up hard. There is no way of keeping unused resin after it is mixed without it hardening.
Paint a thin coat of the mixed resin on the portions of the fuel tank which are going to be covered with cloth. Then set aside and allow to dry. Drying time will vary with the temperature and ventilation. When set outside on a warm day it will dry within a half hour or less, but inside and in a cool place, it may take several days.
After this first coat is set up, apply a very heavy coat of mixed resin, and immediately put the cloth on, working out all air bubbles and making it lay flat. Then immediately completely saturate the cloth with more resin. You'll probably have to use your fingers to get the cloth worked down. Brush cleaner will clean your hands. You must be sure the cloth is thoroughly saturated and laying flat.
After this second coat has set up and dried, apply as many coats as you feel necessary to get a nice smooth job, allowing the resin to dry between each coat. You will want to apply enough coats so that you cannot see or feel the texture of the cloth.
After the last coat has completely dried you will want to sand or grind off any rough spots and so forth. Also, feather in the edges where the fiberglass ends. After giving the whole tank a coat of paint, none will ever know that this is not original. It saves a lot of getting ready and cleaning up if you can do more than one at a time.
Whenever I have to make a new fuel tank, I always fiberglass the inside bottom and half way up the sides before I solder the top on.
Perhaps some of you have done this and it is nothing new to you - however, I have mentioned having done this to several fellows and they had never thought of it and many were not aware that it could be done.
Hope this will help others with the gas tank problem as it has helped me.
Just one last note of caution - this resin has quite a strong odor so you'd better not try to do this in the house; if you do, your wife may throw you, fuel tank and all outside.