A:  One of the central ironies of boating is that we put great effort into making a vessel watertight, only to drill holes through it afterwards! Sealing up fastener holes is crucial to maintaining a dry boat. To prevent leaks through deck fitting fasteners, ensure there is sufficient material at all possible points of water entry.

Sealants are strongest when they are relatively thick. If the layer is too thin it will not be able to stretch as far as a thicker layer would before the bond between the sealant and whatever it adheres to fails.

A trick that gains sealant thickness where it counts is to chamfer the edges of the holes in the deck. This allows a thicker layer of sealant to form at the joint between the deck, fitting and fastener, and goes a long way to ensuring long, leak-free service. Always dry-fit the fitting and bolts, and check that all will fit properly and the bolts are long enough. Pulling the fitting and fasteners after applying sealant creates an awful mess!

As for how to put the sealant on the fastener, there is no hard rule. If you put plenty of sealant on the fitting before positioning it on the deck, the sealant will squeeze into the holes as the fitting is pressed in place. Use a caulking gun to put a dab of sealant on the surface that the head of the fastener will bear against, then push the fastener through the fitting and deck. Be sure the threads of any thru-bolts have sealant on them, too. Add it before placing nuts and washers if needed. Here is a simple way to gauge if you have enough sealant: if the sealant squeezes out around the perimeter of the fitting at all points before the fasteners are tightened, you have enough.

A common practice is to install the fitting with bolts or screws only semi-tight. Then allow the sealant to cure (overnight or according to manufacturer’s recommendations) before tightening the fasteners. This squeezes the rubbery, cured sealant slightly to make an even better gasket. Always allow the sealant to cure before placing any working load on the fitting!

A real challenge is cleaning up the squeeze-out and inevitable smears that are left behind. Using masking tape where possible certainly helps. Make sure to use the recommended solvent sparingly. You don’t want liquid solvent penetrating into the sealant and destroying its cure or mechanical properties.

And the last word, as always: Follow the manufacturer’s instructions to the letter!

For more on this subject, including the use of epoxy fill, see “Repairing Deck Leaks” (October 2006). —Eds.