Caulk and Sealant Test

How does a discerning boat owner decide what's best to use for bedding and caulking, and what products make the best sealants? Here's our start at answering that question.

Caulk and Sealant Test

]In olden days, landlubbers used tons of putty to caulk and seal. The putty was made of whiting (calcium carbonate) and boiled linseed oil. Substitute red or white lead for the whiting, throw in a little hair (any old kind) and you had something called “lute” for pipe fittings. Lute can mean a musical instrument that looks like a bulbous guitar, something valuable, or, as used here, pipe-joint packing.

Sailors always scorned the putty that was used ashore, mostly because it’s not very substantial. It dries out and cracks, even when carefully painted.

For ships, sailors used oakum, which was pine-tarred rope or cotton waste, and a caulking (or chancing) iron and mallet to drive the oakum into the seams and cracks. Modern oakum is made of jute impregnated with something called Bentonite, which makes the stuff expand 10 times in size. (You can still buy oakum. We found it at Jamestown Distributors for a price of $29 for five lbs. British-made Bristol Sussex caulking cotton, $8.40 a lb., and three sizes of chancing irons, each about $20; you’d have to cobble up your own mallet.)

Then and now, it’s both poetical and musical to see a good caulker at work—his mallet swinging rhymthically in one hand while the other hand, the one with a thumb like the handle of an oar, is busy-busy-busy at several tasks. In a boatyard, a caulker ranked just below the top-ranked rigger. Their lofty rank (both topped the carpenter, sailmaker, boatswain, cooper, and common seamen) had to do with the fact that what they did was directly related to getting home after a voyage.

Putty is still used ashore for window glazing. And the word endures, of course, to describe what you are in the hands of the one you adore. But for marine use, it’s all polymers now.

W. Schwenck Gilbert, the English lawyer turned satirical librettist for the composer, Arthur Sullivan, wrote: “…sisters and brothers and cousins by the dozens….” and you could say the same about polymers.

Polycarbonate is the clear, very strong plastic used often for glasses. Polystyrene is the lighter version that can be colored. Polyester you get in clothes not much favored anymore. Polyethelene is light, part-crystalline stuff used for insulation and packing. Polyvinyl is a cheap (but utilitarian) raincoat—or, as Saranwrap, for wrapping up left-over meatloaf. (In a March 2002 article, Practical Sailor went into the chemical make-up of modern plastics, including formulas for polyester, styrene and an interesting hybrid called “block copolymer butadiene.”)

But the ones called polyurethane and polysulfide—along with distant cousins called silicone and multi-caulk—are what concern us when we want to mount or replace things on our boats—cleats, winches, keels, vangs, radar mounts, port lights, hatches, pad eyes, and stanchion bases, etc.

Polysulfide, an organic compound made of two sulfur atoms and one of carbon, is not much used anymore in exposed applications because it suffers badly from ultraviolet degradation.

Multi-caulk usually is a tough, hard adhesive that lacks the needed flexibility.

Silicone (made of sand) and polyurethane (NHCOO), both inorganic (meaning they are man-made), are more complicated and more versatile. Silicone, which while curing gives off acedic acid (smells like vinegar), skins quickly and is easy to handle.

Polyurethane is much stickier and difficult to handle. Get a dab on the edge of your hand and very soon it’s everywhere; it pays to keep everything pristine, with lots of paper towels, tools and rubber gloves, which you discard after using them. Polyurethane makes what is considered a “permanent” bond.

On a boat, the goal is to make everything absolutely impervious to leaks—which can cause rot and lead to the deterioration of most any material. Besides solid wood (even teak and cedar) and fiberglass, wood-cored decks (balsa or plywood) are particularly vulnerable.

Especially to be excluded is saltwater, one of nature’s most complex solutions. Saltwater contains so many elements that scientists have trouble counting them all. Neutralizing all of them is virtually impossible.

The Experts
PS had extended conversations with a dozen experts in the caulk and sealant industry. As is so often the case, the marine segment of the business is minuscule.

Among the experts who offered valuable information was Steve Mishra, vice president/technology, for Tremco Sealants. Tremco is one arm of RPM, Inc., a company with 7,900 employees, and 68 plants in 17 countries. RPM owns Rust-Oleum, Dap, Bondo, Watco, Pettit, Woolsey and Z-Spar and sundry other enterprises. Another was Steve Paget, technical coordinator for Sikaflex, which claims to be the world’s largest (6,000 employees) polyurethane caulk maker. At 3M, a company whose widely-known “5200” has the aura of a benchmark in marine caulks and sealants, it was Alison Berka-Bennett, one of 3M’s many savvy regional representatives.

All of the experts agreed that sealing a portlight, for instance, is much easier than sealing a cleat attached to a balsa-cored deck, even one with spot reinforcement in the deck lay-up. The portlight does not move, whereas the cleat, which will be subjected to heavy tugging from perhaps multiple directions, must have a more elastic sealant to cope with the forces applied and still remain waterproof.

To test commonly available caulks and sealants, PS collected 23 varieties—not all that exist, by a long shot. If many more had been included, the chart on pages 14 and 15 might have approached the size of China. You might note that this chart lacks the customary column for prices. PS considers these caulks and sealants a minor expense. Depending on whether you buy a small tube, large tube, cartridge or other type of container, the price can range from $6.99 for an unusual 1-oz. tube of 3M 5200 (which by the 10-oz. cartridge is $1.40 an oz.) to less than $1 an oz. for the hardware store varieties. A 10.5-oz. cartridge of Sikaflex 291 is about 78 cents an ounce.

Although price might be a factor if one were caulking siding in a new boathouse, it probably isn’t a consideration with most sailors. Each boat owner will most likely want the product that does the job best, for the longest period of time, period.

The chart does include cure times (which are important because they indicate how much time you have to work on a seam or joint); if the product is paintable; what is specified for cleaning up; if the product is for use both above and below the waterline, and PS’s observations after using these materials.

What To Test For
Our tests were devised to consider the three most important qualities of these products and answer, on a comparative basis, the following questions:

1. Are they really waterproof?
2. How long do they remain elastic?
3. How strong is the bond?

In addition to these three basic attributes, caulks and sealants sometimes are used to electrically isolate the parts joined. In these cases, a lot of pressure shouldn’t be used so that you avoid squeezing the caulk out of a joint.

Caulk and Sealant Test

Another consideration: Above or below the waterline? Silicone is not for use under water, nor is it paintable. Polysulfide and polyurethane are.

As explained by Steve Mishra, the strength and elasticity requirements of any caulk or sealant oppose each other.

And waterproof? It’s difficult to define. Further defining the purpose of caulks and sealants is the question of whether adhesiveness is more important than watertightness.

“We can create and produce caulks and sealants with amazing qualities,” Mishra said, “but combining the various properties is what’s difficult.”

3M likes to pass around the story about someone who asked how to remove something bedded in 5200. The answer was, “Why was 5200 used? It’s a permanent bond.”

3M’s answer was to bring out 4200, which has half the adhesive strength of 5200. (3M’s 4200 is exactly the same formulation as 4000 UV.)

But nobody likes to talk about what happens to any of them when they grow old and brittle, as they all do.

How old?

Who knows? There are in every big harbor lots of 20-year-old boats whose portlights (and other equipment) have never been rebedded.

“Fifteen or 20 years!” said Mishra. “That’s pretty old for a caulk.”

Setting Up The Tests
For this test, which will be another long-term enterprise, with the usual interim reports along the way, here’s how PS set it up.

To test adhesiveness, strength and elasticity, five “surfaces” were prepared. They are, raw aluminum, anodized aluminum, teak, fiberglass, and stainless steel. Twenty-three globs of caulk about the size of walnuts were applied to each material.

In the raw aluminum, stainless washers were embedded standing up. In the anodized aluminum, oak dowels were embedded. The remaining combinations were teak/stainless steel bolts, fiberglass gelcoat/stainless steel bolts and stainless sheet/brass screws. The photo on page 13 should help to make this portion of our test clear.

All 115 globs of caulk were kept indoors for several weeks to cure. Because the globs have considerable bulk, they require much more time to cure than would a thin layer in a joint. As a general rule, 10 days curing time is needed for each eighth inch of caulk.

Only then was the panel placed outdoors in open sunlight, exposed to all weather—winter and summer. Because the globs of caulk are more exposed to sun and weather than might occur in, for instance, a thinner layer used to bed a cleat or toerail, the “aging” should be accelerated.

The panel will be checked monthly, but it may take a year or more to detect significant differences.

To test waterproofness, 25 (two without caulked tops were to serve as controls) small, tall, very clean bottles were carefully scribed with an abrasive tool and filled with water to that half-full mark. The bottle lids were smeared liberally inside with caulk and caulk was “tooled” at 45° against the inside lip of the lids. The lids were snugged down without much force, in order to assure that the caulk was not squeezed out.

The 25 bottles, numbered on their sides and bottom, were placed in a wood rack made expressly to assure that each bottle was equally exposed to sun, shade, hot and cold (including freezing).

The bottle rack was placed outdoors, always on its side, not only to assure that the caulked tops had both air and water contact, but to permit the water to freeze without breaking the bottles.

Constant expansion and contraction, along with some sunlight to work on whatever thin layer of caulk it could reach, will test the caulk’s integrity.

If a caulk is not waterproof, water loss will be plainly revealed by the waterline.

As with the panel, the bottles will be checked regularly—especially in very hot or very cold weather.

The Bottom Line
The objective of this test is not to determine how long any of these caulks and sealants last. As pointed out above by an industry spokesman who expressed surprise that anyone would expect any of these products to last “15 to 20 years,” expecting them to last forever isn’t in the book.

What results are obtained in a year, two years, or more will be comparative—meaning that the tests may reveal which retain their adhesiveness, which remain flexible and, perhaps most important, which are waterproof, and for how long.

What will happen?

PS does not know.

Will some of the caulked globs lose their grip on some or all of the surfaces? This may be measured later, if necessary, by hanging weights on the washers, bolts or dowels. Can those that remain be evaluated by trying to pull them loose, perhaps measuring some (the bolts) with a torque wrench? Will the bottles lose water because the severe conditions surely must facilitate some migration of air or moisture? Will the caulk become hard and inflexible, and subject to cracking?

Hard to say, but eventually—and intermittently along the way—we’ll develop some useful data for readers. Stand by.


Also With This Article
“Spec Sheet: Caulks and Sealants”
“Prior Testing”

• Ace Hardware,
• Boat Life, 800-382-9706,
• Elmer’s, 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