Sticky Situation

After a year of testing, the proof is in the goop.


About 16 months ago, Practical Sailor kicked off a prolonged test of 23 caulks and sealants, which are so important to boat builders and owners. As shown on the chart (see sidebar), the products included silicones, polysulfides, polyurethanes, acrylic latexes, vinyls, polyethers, and some combinations thereof.

Sticky Situation

The brands mostly are those strong in the marine field-like 3M, Sika, and BoatLife-but also include hardware-store varieties like Elmers, GE, and RPM products.

The testing was planned to answer the three most important questions about these products:

1. Are they really waterproof?

2. How long do they remain elastic?

3. How strong is the bond?

To the equally important question of how long they last, PS can only refer again to the industry expert quoted in the initial report published April 1, 2005.

When informed that there are many boats built in the 1970s and 1980s that have never even had their portlights recaulked, he said:

“Fifteen or 20 years? Thats pretty long for a caulk.”

He suggested strongly that there are lots of boats overdue for recaulking and rebedding. However, if you follow the old adage, “If it aint broke, don’t fix it,” you can wait until something leaks, and hope you catch it in time to avoid any damage…like discoloration, delamination, etc.

Because it seemed unlikely that any one product could be the most waterproof, the most elastic, and the strongest adhesive, and because not all those attributes are called for in all applications, the PS test was somewhat complicated. It was designed to help a boat owner select the caulk or adhesive that best matched the task at hand-whether overhauling leaky portlights; rebedding a cleat or winch; or sealing in place a new hatch.

Because some readers faced with immediate problems asked for interim reports, PS published in the Nov. 15, 2005, issue a report dealing with waterproofness and short-term elasticity.

Elasticity is desirable if great force is to be exerted on whatever equipment is involved. For instance, a cleat used as a sheet anchor is better mounted with a caulk that has long-term elasticity; a caulk that turns hard and brittle would be more likely to crack and start leaking. By contrast, a portlight or hatch should be mounted with a caulk that has great adhesiveness and is very waterproof.

For this “final” caulks and sealants report, elasticity was graded with deflection measurements on the many combinations of all the materials tested-wood, aluminum, fiberglass, stainless steel, and brass-with the 23 kinds of caulks and sealants.

As expected, the half-inch oak dowels best resisted bending, and the thin stainless washers were the most movable (See “A Long-term Test.”). The comparative measurements proved to be consistent, meaning that the percentage differences between fiberglass, wood, aluminum, etc., were the same. So, fine measurements were done on the quarter-inch stainless bolts-primarily because the bolts were the easiest to scale accurately.

Subjected to equal pressure from the side, the range of deflection on the stainless bolts was considerable-from “not measurable” to 5/8 inch. (When deflected 5/8 inch, a bolt is about 30 degrees out of line.)

The ranges were broken into four groups, which are rated on the chart as most elastic, fairly elastic, a little elastic, and not elastic.

The only notable surprise in this elasticity test is that not all silicones were equally elastic, although they are more flexible than the polyurethanes, which were generally the least elastic of all the materials.

The greatest anomaly involved two silicone/acrylic caulks-Elmers Squeeze N Caulk and RPMs Dap. The Elmers was among the most elastic. The Dap (often used for bathtub showers) cured hard.

After a years exposure, the greatest elasticity was shown by Sikas 295 UV and the 3M Silicone.

As noted in the initial report and update, the waterproofing test involved 23 small bottles half-filled with water, with the cap threads sealed with the 23 products. (Two extra bottles, one very strongly tightened, the other firmly tightened, were included as controls. On the inside of the caps are thin rubber seals.) Because it was difficult to put exactly the same amount of water in each bottle, the water level in each bottle was marked precisely by scribing the glass with a carbide tool.

The bottles, placed in a wood rack to make their exposure exactly the same, were turned frequently-upside down, on their sides, etc.-during the one-year, all-season test period.

As shown on the chart, one bottle lost all its water in less than a month; however, PS considers that instance (in which the sealant was something called “Goop” made by Eclectic Products) to be an aberration, probably caused by faulty handling.

For the other 22 products, there was only very slight loss of water in six (see chart) of the bottles; there was no detectable loss of water in 16 of them. It may be a minor point, but it suggests that if a great seal is what you want, select one of the 16 that lost no water.

The final one of the three qualities tested, adhesiveness, was the most difficult to judge. None of these caulks and sealants behaves like glue of any kind-animal, vegetable, chemical, or epoxy.

After some initial experimentation on a separate test panel made up expressly for that purpose, as well as cross-checking with different combinations of the materials, PS determined that breaking loose the stubby oak dowels provided the best indication of how tight a grasp each product had on the anodized aluminum bar. The force necessary was measured with a simple but accurate spring scale.

The adhesiveness was graded as “Excellent,” “Good,” “Fair,” and “Poor.” Six of the 23 products were superior.

The “best” (as shown on the chart) was Eclectics Goop, a hardware store variety of contact adhesive.

The other five (shown as Excellent on the chart) are Ace Hardwares Clear Sealant; Sikas 291, 291 LOT, and 295UV; and 3Ms 4000 UV.

The Bottom Line
Although seemingly complicated, the qualities of these caulks and sealants can be scanned easily on the chart to determine which caulk might be best suited to the task to which you set it.

As mentioned before, this was not a good, better, best test. It was designed to measure and compare the 23 products behavior to emphasize waterproofness, elasticity, and adhesiveness.

A further note: Although this was an attempt to show how the products “aged,” the time period was but one year. Prolonged aging, perhaps five to 10 years, might yield different results.

From a practical point of view, one might combine and review the three judgmental columns on the chart in the hope that they might reveal the best all-around caulk/sealants. Our recommended products are: Ace Hardwares Clear Sealant, Sikas 291 LOT, Elmers Squeeze N Caulk, and West Marines Silicone.

However, conservative, logical thinking suggests that such a generalization might not be warranted and that it would be far better to match the product to the job, even if it is a bit of a bother.

Ace Hardware, 866/290-5334,
Boat Life, 800/382-9706,
Elmers, 888/435-6377,
GE/Marine, 800/626-2000,
Eclectic Products, 800/767-4667,
Gloucester Co., 800/343-4963,
RPM Inc., 330/273-5090,
Sashco, 800/289-7290,
Sika Corp, 201/933-8800,
3M Scotch Brand, 888/364-3577,
West Marine, 800/262-8464,

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at