Make sure all the rust is gone.
Cut two or more sheets of fiberglass CLOTH to fit your needs. Fiberglass comes in a fluffy loose mess or a woven cloth. Use ONLY the cloth.
The cloth coverage must extend well above the highest point at which you have any leaks. Does not matter if you have to make notches in the cloth to allow for curves/corners. Do it. Overlap as needed, but leave no spaces, holes or blank areas of any size. You can sand it smooth later.
Sand the bare part, then mix up a liquid epoxy according to directions on the can, brush it on liberally so that there are NO bare spots, and lay on your first sheet of fiberglass. Pull it taut, then use the brush with more resin to splay out the cloth. The extra resin will fill all cavities. Turn the tank upright, and wait for a few minutes until it starts to set up. Turn it once or twice to minimize runs and keep the resin consistent in thickness.
After that, ignore the runs.
It is critical that there be NO bare spots on the metal. Anywhere there is metal, the fiberglass must stick to it.
At this point it is preferable to let it set overnight, sand smooth all over (and remove all drips), then continue.
It may be possible to place a second sheet before the first batch of epoxy starts to set, but this will depend on your experience and knowledge of fiberglass. If you have little or none, stop after one sheet and wait overnight.
Trying to rush a second coat/application is just as good as throwing stones at it....all you will succeed at doing is destroying your first effort.
Coat everything with resin and lay on a second sheet. Again use the brush to both stretch it smooth, and fill in the cloth completely with fresh resin.
There will again be excess resin to drip away.
When hardened, sand away the drips.
If the tank had no major holes, a third sheet would be preferable, but two should do the job, provided you were liberal enough with the resin. One sheet may look just as good, but one is NOT sufficient.
If the tank had major holes, then a third sheet is necessary.
You can use a spray paint can to get close to the original colour.
Using suitable gasket material, remount your petcock AND add an in-line fuel filter, fill the tank and drive on.
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