Dried or cracked bedding compound under a boat’s decking can cause leaks; this “doughnut” solution is the perfect fix.
If your boat is more than a few years old, you’ll eventually need to renew the bedding compound under at least some of her deck hardware. This chore is important, especially on boats with balsa-cored decks, teak-planked decks and fuel tanks located directly below the fuel fills. Water intrusion under such circumstances can cause rot, corrosion and other dire developments long before obvious warning signs manifest. Probably the gloomiest (read: most expensive) problems arise in a fueltank-directly-below-the-fuel-fill scenario. If the bedding compound involved is dried out and cracked, water is undoubtedly slipping through, damaging the surrounding coring material (if the deck is in fact cored) and eating away at the top of any fuel tank that happens to be blanketed with a moisture-trapping, crevice-corrosion-causing material, like sound insulation, plywood sheathing or plain ol’ fiberglass. Steel tanks are notorious for this kind of thing, but modern aluminum tanks are not immune either.
How do you deal with the issue? Creating a thick, evenly compressed, gasket-like “doughnut” of polysulfide compound material under the leaky fuel fill or fills is the answer. And, to guarantee the success, I suggest avoiding the use of simple screws when securing the fill once the new polysulfide is in place. Instead, opt for bolts, a backing plate fitted from beneath the deck (3/8-inch starboard works well, with a large, central opening cut with a hole saw), stainless-steel fender washers and aviation-style locknuts.
Does this belt-and-suspenders approach add difficulty to what might otherwise be considered a routine task? Absolutely. But it also makes for a much more positive, watertight, long-lasting connection between your boat’s deck and the deck fill that penetrates it. Moreover, the use of bolts, nuts and a starboard backing plate—as opposed to three or four separate, wholly independent screws—creates a stronger, more evenly distributed, unified bond between all the surfaces involved.
And don’t forget: At first, only lightly tighten the aviation-style nuts under the deck. Overtightening before the polysulfide has cured and hardened will squish too much of the stuff away, thereby eliminating or at least abating the gasket effect you were shooting for in the first place. Once the compound has set, however, go ahead and tighten things up with a robust tug.
Finding a perfect replacement is not often easy.