I tried to fix a bad seal on a fuel sending unit located on the top of
my aluminum tank. It didn't work, so I took it to three different marine
mechanics, one of whom tried three times to fix it. After taking who
knows how many gallons of gasoline out of my bilge because all of these
failed, I tried "Plan B."

I covered the whole damn thing with a pile of grey Marine-Tex after
checking with the company about its compatibilty with gas and aluminum.
It looks like some animal took a **** right on my tank. I feel guilty
about such a repair and know I'm never going to be able to get that
sender out, but I don't have any hint of a leak anymore. I use a fuel
management system to figure my consumption anyway, and i have a
gasoline/CO/propane sniffer.

A 3" hole is too big for such a repair, but if you made up a 5" plate
with your fuel connector, drilled some holes, put down a gasket and then
sealed it all up this way, it might work for you. Be careful with sparks
while doing all this.

By the way, I have absolutely no qualifications as a marine mechanic
whatsoever to recommend anything like this to anyone, so if you blow
yourself up because it leaks someday, don't say I didn't warn you....