How to Choose the Right Sealant

Passage Maker - - Contents - eve Zim­mer­man

Here’s a scene that plays out in the boat­yard from time to time. A work or­der calls for rebed­ding a leak­ing port­hole. Af­ter all the fas­ten­ers are re­moved, the hard­ware re­mains firmly in place. Some coax­ing with a putty knife and some heat fails to free the port­hole from the sur­face. At this point we can iden­tify our foe. It’s a fa­mil­iar an­tag­o­nist known com­monly by a num­ber: 5200 (of­fi­cially it is 3M™ 5200 Ma­rine Ad­he­sive Sealant). Out come wedges and ham­mers and sol­vents and curses and grunts. Af­ter an hour or two of strug­gle, the port­hole fi­nally lets go of the cabin side, but not with­out break­ing off chunks of gel­coat and fiber­glass. This un­for­tu­nate sce­nario begs this ques­tion: If the port­hole is held in place with stout stain­less steel ma­chine screws, why does it need an ad­he­sive to hold it in place?


First you must un­der­stand what you are try­ing to do. If you are try­ing to hold a fit­ting in place, you need an ad­he­sive. The hullto-deck joint would be one ex­am­ple; a fiber­glass-mounted pad at­tached to a cabin top would be an­other. And if you want to hold it in place and keep wa­ter out, you need an ad­he­sive sealant.

If you are try­ing to keep wa­ter out of a seam or a joint, you need a caulk. Caulk­ing takes place af­ter the item has been in­stalled. Fill­ing the seams be­tween teak planks on a deck calls for caulk­ing, as does cov­er­ing a joint be­tween a cabin top and a cabin side with a bead smoothed over the seam.

If you are try­ing to keep a fit­ting from leak­ing and/or to keep wa­ter from stand­ing be­tween the fit­ting and the mount­ing sur­face, you need a sealant. Ex­am­ples in­clude a lad­der base mounted on the cock­pit sole or a port­hole mounted on a cabin side.

The sealant should be ap­plied lib­er­ally so that you see “squeeze out” of ex­cess ma­te­rial around the en­tire perime­ter. Two-inch-wide tape helps keep the ex­cess com­pound from mak­ing a mess.

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