Originally Posted by Ice-breaker
I would go with the rubber gasket approach suggested by Raz, as it gives you the flexibility to open and close your tank at will without having to remove and replace caulking or other sealant.
That's the whole point of installing the deck plate.
The top of my tank wasn't completely flat. I suppose a soft rubber would have worked. I already had the 3m 5200 because I was installing bulkheads in my kayaks. I put a hose in the filler blocking most of the opening, cracked the deck plates loose until all air was gone and water came out the top, tightened them, then let them sit under pressure from the hose for a few hours. No leaks