If you don't plug the hole when you solder you need fuel in the float so that when you do the final check in boiling water it can confirm that the solder has plugged the hole. If there is no fuel in the float to evaporate and bubble out the hole you won't know if its fixed.
You don't need fuel in the float. Fix the hole and submerse the float in hot water, the heat will expand the air and bubble out if you still have a hole. The solder required to repair it will not significantly affect your gauge calibration, the difference in SG of the fuel between a hot and cold day would have more affect not to mention the crude calibration it had originally.
Firstly, what about something like the fuel tank treatments POR and the like make to treat a rusty tank and seal it up. Would that seal the outside of the float enough to stop fuel getting in... I'm not sure just throwing it out there.
Secondly there might be some kind of fuel safe sealer/silicon you could use just to seal up the hole. On that though be particularly careful to make sure it is fuel compatible. Dad had new fuel tanks put in his boat and where they sealed the filler into the top of the tank with sealer we questioned its use being exposed to petrol the whole time and where told it was fuel compatible. Anyway long story short we chased engine starving problems for ages and then finally found small bits of sealer had broken down and fallen into the tank where the fuel made them swell and they would bloke the fuel lines at high revs when the pump was really sucking but then when you backed off they where semi buoyant enough and the sloshing around of the ocean would move them away from the pick up and the engines would fire up and run perfectly again.
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