3m 5200 is certainly good stuff, but people seem to think it is unable to be removed once applied. This is far from true. I;ve used a lot of it and removed a lot of it. Compared to regular caulk it is more difficult, but by no means impossible, adn in reality not all that difficult. Backing a potted screw out of it, is simple, once one gets access to the screw head. Couple seconds with a wire wheel and a dental pic.
I think Naio's best option it the waterproof JB weld. Do your best to mix it properly, ratio wise, and completely and perhaps get a Q tip and some DNalcohol inside the threads.