The Right Caulk for Your Boat

Posted by Darrell Nicholson at 03:29PM - Comments: (4)

photo by Ralph Naranjo
photo by Ralph Naranjo

The seven-product test field included Sika’s Sikaflex 291 LOT (top) and (left to right) West Marine 8200, 3M Silicone, BoatLife Life Caulk, 3M Fast Cure 4200, West Marine Multi Caulk, and 3M Fast Cure 5200.

Application ease is an important factor when it comes to the caulking and seam-filling process, and some sealant/adhesives behave better than others. Among the most desirable traits is viscosity; the ideal consistency should be a paste-like material that can be easily injected into a seam or spread on a piece of hardware.

Of the products we tested for our test of sealants in August of 2010, we found that Sika-flex 291 LOT and 3M’s 4200 Fast Cure had the right consistency to make application a user-friendly experience. They were thin enough to penetrate small seams, but stiff enough to form an even bead.

When applying sealants, surface prep follows the painter’s mantra of smooth, even, and clean. Any contamination—be it oil, wax, or loose varnish or paint—has the ability to nullify the adhesion of a sealant. The prerequisite to a reliably caulked seam starts with the usual scrape, sand, dust, and wash clean.

Most adhesive sealants like well-prepped gelcoat or an epoxy-primed surface, but in the case of the latter, beware of what’s called “amine blush,” a residue that forms on the surface of epoxy resins and primers when they cure. Washing the surface before applying an sealant is a simple cure that removes the adhesion-robbing residue.



This table, adapted from marine maintenance guru Don Casey’s second edition of “This Old Boat” (McGraw Hill, $50; also available as iPhone app and eBook) offers some guidance as to which type of adhesive/sealant is best for specific bonds.

All products we tested had the near-magic ability to stick to every place they are not welcome, and attention to detail during the application process is required. Lightweight throw-away gloves, a roll of paper towels, and a paper bag to dump contaminated wipe-off towels into are a must. Work from the top down whether it’s on deck or filling plank seams on the hull. Use masking tape to confine the over-spread and maintain a crisp cut line. All but silicone sealers can be sanded and painted, but the best bet is to achieve a fair, even bead that needs no hiding.

If you’re left with a half-used tube of sealant or caulk, try this preservation method suggested to us years ago by former contributor Dick Wilkens: With a piece of plastic (like Saran Wrap) over your finger, push the sealant back into the tube a bit, leave the plastic in place, and replace the tube’s cap. The plastic excludes the air that usually lets the sealant harden under the cap. According to Wilkens, this can extend the usable life of the product for years.

For more help on selecting an adhesive sealants, subscribers can refer to our August 2010, April 2005August 2006 and November 1998 test reports online. If you are sealing hatches or ports, you will also want to read our December 2012 PS Advisor column, which introduces some other options for bedding and sealing. There is also our October 2008 test of caulks for teak decks.

Comments (4)

Bostik / Simson MSR Deck Caulk Plus

I read your very interesting article on the 8 months teak caulk tests:

We used this product on a cord deck, was told that we did not need to prime the grooves first as it sticks to cork very well.

We have had a very bad experience with the Bostik / Simson MSR Deck Caulk Black - It has melted and seems not to be able to handle UV at all. It has melted out of the grooves and is all over the deck. Its supposed to handle temps ranging -40 deg to 100 deg.

Am I correct in saying that only chemical testing was done, not UV?

BTW - The white cannot tolerate UV at all either.
I was wondering if anyone else has had the same problem?

Posted by: Jenson | July 29, 2016 7:01 AM    Report this comment

Above or below the waterline usage is also critical in selecting a suitable sealant. This should have been addressed. Also silicone contaminates a surface that no solvent can effectively remove. Sanding appears to be the only remedy. Butyl tape is still the champion for bedding port lights and deck hardware.

Posted by: Raymond S | April 20, 2015 8:54 AM    Report this comment

Good old fashioned "rope caulk" -- the brown stuff -- while getting harder to find, is great in some applications because it never dries out or gets hard. It's also relatively cheap.

Posted by: james s | April 17, 2015 11:28 AM    Report this comment

I would love to see a review including some of the very economical polyurethane and polysulfide products at Lowes and Home Depot: Sikaflex and Locktite PL seem to offer performance right up there with big name marine products for 1/3 the price. I've been using them for years, right alongside 3M 4200/5200 and the like.

Posted by: Drew Frye | April 15, 2015 5:56 PM    Report this comment

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