I recently restored my cub. After bring it back
from the show this morning i noticed two
bubbles in the paint of the fuel tank. Under
each one was a screw along with brazzing on
one. I believe the vapors are coming through
by the screw and eating the paint and filler.
What should i use to seal these up? A epoxy, JB
weld, brazzing? Something else? Thanks ahead
of time!

Repair your tank properly,have it welded or brazed by someone capable of cleaning and removing gas fumes,then repaint.Its always better to do it right the first time

jimmy

thats right, 90% of the work is involed in preparation for paint. painting is the easy part.

The answer is solid repair. Removing all fuel and removing the failed filler with hand tools, not power driven will get down to the screws. remove them and finish cleaning. Take the tank to a repair shop that can braze in a real patch.
Remember filler has a different expanson and contraction rate than steel. thick (more than 1/8th inch are going to crack and remain a problem. JB weld seems to be a better option, as do epoxy materials designed fro fuel tank repair. Then shape, prime and finish. Remember, heat drives fumes in an empty tank to the explosive range, even an electric soldering iron will make trouble. (it would need to be massive to heat the tank to solder melting temp anyway)
Jim

I would have fiberglassed it and that would have never been a problem

No plastic applied from the outside is going to hold. Clean it down to bare metal and repair with silver solder. Be sure the tank is completely empty, dry, and free of fumes before you start. Put some water in it to be sure.

Best bet would be to take it to a local radiator shop & have it tanked, repaired when you get it back prep & repaint. Nothing you can purchase will hold after that type of "repair", a screw & bondo.....let me up !!

I'll share what I did with my F-12 tank. It too had 2 screws and bondo on the bottom for a repair.

I emptied the tank and removed ALL fittings, caps, etc. and I also removed the repair. I then let the tank sit in the hot sun for a few days, which "cooked" out all the fuel that was in it.

Tank was cleaned and paint/rust was removed both inside and out. Repair area was rust stabilized and then filled in with JB weld, and sanded/formed to contour. I then used an epoxy tank liner on it (following the directions). prepped, primed, and painted.

No leaks, no peeling, and no bubbles. for smaller repairs to sheet metal and gas tanks (dents). I prefer JB weld vs. bondo. It is more expensive, but I feel it is a better solution for parts that sustain lots of vibration. The only body filler product I use is spot putty to fill any tiny bubble holes in the JB weld and fine feathering. I would be surpised if the spot putty was thicker than 1/64" in any of my applications.

there are good suggestions in this thread. Use the one you are most comfortable with doing. Brazing, solder, MIG weld are all viable options as well. It all depends upon what your capabilities are and the task at hand.