Difficult for anyone that does not have an O'34 and/or has been involved with it's
fuel tank to comment very meaningfully on this question without a few details. It also makes for much more interesting reading for the helpers and the lurkers.
Exterior patches are probably not approved for survey/insurance purposes but as a practical matter,
epoxy putty or 3M5200 seldom lets loose once applied to a clean dry roughined surface. Anouther concern would be that if the corrosion is exterior, is there just one spot, and if it is an interior corrosion leak then there are likely other spots that are thin and could leak at any time.
(1) What sort of leak,
(2) How was the leak discovered.
(3) What tests have been performed and/or repairs attempted so far.
(4) Have hoses, vents been checked.
(5) What material is the tank made of.
(6) Where is the tank located and what are the obstructions to removal.
(7) What size (gallons/dimensions) is the the tank.
(8) What is your ability/knowledge as related to such repairs.
(9) Picture is worth 1000 words.
Best of Luck...