... I have the 4" Beckson Screw Out Plates mounted to my custom polyethylene water tanks. I tried silicon (I generally don't like to use this on the boat) and they leaked a bit. Then I tried LifeSeal, I put down a bead, tighten up the thru-bolts, then the next day tightened them another 1/4 turn or so, and some still leaked a bit. The leaks are at the tank to Plate joint, not around the fasteners. I suspect when I fill the tanks too much, there could be excessive head pressure, or perhaps with the water banging around when conditions are rough. My tanks are well baffled.

Looking for suggestions. I think the silicon and the LifeSeal simpy did not bond to the Poly, so I'm not sure any sealant will, 5200 or other (and I generally don't like using that stuff either).

I think I will next try to gasket the Plates with some kind of neoprene and see how this works ...

thanks, cheers, John