Suffering from sealant confusion?

By Scott Thurston

Article taken from Good Old Boat magazine: Volume 3, Number 2, March/April 2000.

Your job requires a sealant. You don’t have to be a chemist to choose the right one.

Sealant supplies on the shelf

In my experience, there are always two things trying to get into your boat
that you don’t want there: water and your annoying brother-in-law. While
there’s not much you can do about family problems, there is something you
can do about the water.

The old saw describes a boat as a hole in the water, but the reality is
that your boat is full of holes. Most of them were put there intentionally:
to drain rainwater or to let water flush through, to cool a motor or to
flush a head. But uncontrolled water is the stuff we worry about. It’s what
rots our balsa-cored decks, drips on our bunks on a rainy night, or causes
the boat to sink when we’re not paying attention. It can make us uncomfortable,
cost us more money than we want to spend, or, ultimately, destroy that which
we’ve worked so hard for and on.

Any time a hole is drilled, even partially, through a solid structure, it
provides a place for water to enter. That hole is probably at least partially
filled, either by a screw or a piece of hardware. The problem is that the
two parts, shaped to their own purpose, have different shapes, and it’s
rare for the two parts to match exactly. What is needed, whenever two or
more solid objects are fastened together, is something that will keep even
the smallest amount of water out, something to seal the two mating surfaces
and dam the trickle before it becomes a flood . . . in other words, a sealant.

The idea is not new. Historical records show that early boatbuilders were
confronted by the same problem. The rediscovered vessels of Native Americans
and Vikings show evidence that thickened organic materials were used in
the construction and repair of their hulls. This was usually a pitch- or
tar-based material, derived from the same sources as their other building
materials. They first used sap from trees, boiled to thicken it, and later
used steeped and rendered animal by-products, such as cartilage and horn.

Eventual decay

“Stockholm tar” is still fondly remembered mostly for its pleasant aroma.
This was used to waterproof hulls and rigging back in the days of wooden
ships and iron men. These old products worked reasonably well and (within
the technology of the day) demanded no more maintenance than did the rest
of the vessel. But because they were organic, they would eventually dry
out or decay. The basic families of these mastics are still available, even
today, under the generic title of “bedding compounds,” though their chemistry
has become a little more refined over the years.

But boats and society became more plastic, and boaters demanded more use
for less maintenance. Other industries, in particular the aerospace industry,
were developing materials to seal panels that made up flight surfaces and
withstand the rigors of flight and space travel. As the performance level
of boats increased, as speeds grew larger from more efficient engines and
lighter but stronger materials, the stresses and vibrations grew correspondingly.
Naval architects, who had looked to the aerospace industry for new building
materials such as glass-reinforced plastics and acrylics, also looked there
for materials to seal them.

Today, we have available a whole range of sealants and, though they are
designed to do much the same job, each has its own purpose. Some are quick-curing,
for convenience and to aid in the economics of construction. Others have
high bond strength, both to the part to which they’re applied and internally,
so the adhesion won’t break along the glue line. Over the last several years,
the distinction between sealants and adhesives has become blurred, as greater
experience with the chemical combinations bridge the gulf between the historical
tradeoffs of quick cure and strong bond.

Three families

Silicone sealant tube

Today, there are three basic families of chemicals used in marine sealants:
silicones, polysulfides, and polyurethanes. Recent advances in epoxy technology
are starting to increase their utility in boat construction, however. (See
sidebar on Page 15.) All of them are desig ned to adhere to a surface, cure,
and remain flexible. By doing so, they accomplish three things: they form
a water- and air-tight seal between two or more surfaces; they help join
the surfaces together, often with the aid of mechanical fasteners; and they
isolate the surfaces, to help prevent the passage of noise or electricity.


Silicone is perhaps the most basic of the three. It is created from combining
silica, one of the most useful of the industrial chemicals we have and one
of the most common building blocks of Planet Earth, with a variety of other
organic compounds. The stuff we squeeze out of a tube is chemically similar
to the spray lubricants we use, and we also find it in paints, waxes, and
other protective coatings and in electrical insulation. Common in household
and automotive applications, it is among the first things we think of when
we use the term “sealants.”


As the name suggests, polysulfides are derivatives of the element sulfur,
another common earth element. Early in this century, there was an explosion
in the use of sulfur for industrial and medicinal applications as the particular
properties of the element were explored. Heated nearly to the boiling point
and then immersed in water, it forms a clear, sticky material that, when
combined with other compounds, has high adhesive properties and is not particularly
affected by long-term exposure to moisture. It’s easily identified by its
strong aroma and good tooling properties.


#m Marine adhesive

Like the other two sealants, polyurethanes are blends of other materials
in a base, this time urea. Urea is a naturally occurring by-product of metabolism,
though now mostly created synthetically. (When you read about slaves harvesting
guano from remote, bird-populated islands a century ago, it was the urea
content they were after.) It’s an acidic compound used in fertilizers, among
other things, whose elements combine hydrogen, nitrogen, carbon, and oxygen
into long molecular chains, longer than the molecules of silicones and polysulfides.
These longer, more complex chains mesh within each other more intricately,
and this gives them greater bonds within the sealant and to other materials.
As a result, they have greater shear strength than the other two families
and are less likely to break along a glue line.

Since the job is the same, why are there so many different types of sealants,
and why should I use one and not the others when I’m working on my boat?

The simple answer is materials. What am I trying to keep from leaking, what
is it exposed to, and what am I trying to bond to it? A modern boat is built
of many materials, and the variety of coatings and other chemicals with
which it may come into contact are just as varied. The specialized world
of chemicals being what it is, not all of them work as well together as
they might. Think of the different boats around you in your marina. They’re
made of aluminum and mild steel, ferroconcrete, wood (both raw and resin-coated),
carbon fiber and graphite, ABS and polyethylene plastics, and fiber-reinforced
plastics – whose bonding agents can be polyester, vinylester, or epoxy products.
And after the hull has been created, it can be finished in gels or paints
in any range of chemical combinations, from lacquers and enamels to urethanes
and polyurethanes.

Diverse hardware

Life Seal for fiberglass

Hardware materials are just as diverse: aluminum, zinc, bronze, and stainless
steels are some of the metals, and there are alloy choices in each of those.
Some of the hardware can be plastic, either nylon or reinforced ethylene.
Portlights can have tempered glass in them, or Lexan, or Plexiglas, and
frames of ABS or metal with rubber or vinyl inserts for linings. Other chemicals
are found on board: fuels and oils, cleaning solvents and waxes. The solvents
and characteristics of each of the families of sealants have properties
that make each one better suited to a particular kind of work.

Silicones, for instance, are very flexible. They will “give” in response
to slight shocks and vibrations without tearing or separating from the substrate,
and they tend to be chemically impervious. Fuels and oils won’t break them
down as fast. They are chemically compatible with most plastics and are
very good at creating a dividing line between electrically dissimilar pieces.
The cure rate is fairly rapid, too, and in most cases the material will
be tack-free in a couple of hours.

The down side is that, though they are elastic, most silicones don’t have
much bond strength, particularly to fiberglass, and won’t do much more than
hold themselves together. The prudent boatbuilder won’t use them for much
more than a gasket material and will depend on mechanical fasteners for
strength in part assembly. You can’t sand or paint them; in fact they can
contaminate a surface that might be painted or glassed, and their bond to
raw wood is not very good. When they are applied in areas of high heat,
they can over-cure and become brittle, causing them to fail sooner. Purer
forms of silicone can allow mold to grow, and this can contribute to rot
in wood (as well as a decline in aesthetic appeal) although most used in
marine applications have a mildicide mixed in. A marine-grade silicone also
differs from household-use silicone in that it has inhibitors to protect
against ultraviolet light degradation and tends to retain its flexibility
much longer in the marine environment.

Twenty-year joint

The polysulfides, such as BoatLIFE’s Life Calk or 3M’s 101, bond to wood
better than silicone does and also bond fairly well to metals and fiberglass.
However, they can melt some plastics and acrylics, such as Lexan, and some
vinyls can become softened by exposure to their solvents. Because of the
metallic nature of sulfur, they are not particularly suited to electrical
insulation and should not be used between two items of dissimilar metals.

Polysulfides are not as elastic as silicones and shouldn’t be used where
constant vibration will occur while the part is under stress or expected
to flex too much. They can be affected by common shipboard chemicals, though
degradation doesn’t happen immediately to fully cured products. Cure times
are more dependent on temperature and humidity and generally can take one
to three days to cure completely, but you can sand them, and they won’t
contaminate a surface to be painted. In fact, they will retain paint once
the solvents have evaporated. With luck, a polysulfide joint can last 20
years or more, and there are many good old boats out there keeping their
integrity because of polysulfide sealants.

Polyurethanes, though they’ve been around for a while now, are the most
recent development of the three and were created to seal and join laminated
panels in the aerospace industry. As a result, they bring extreme adhesion
to sealants and bond well to most modern boatbuilding materials. They are
best exemplified by 3M’s well-known 5200, or Sikaflex 292. (There is a story,
apocryphal in the industry, about a 35-foot sailboat hull being loaded by
a crane onto a trailer. After the hull had been lifted into the air and
settled on the truck, someone noticed that the keelbolts had not been fastened
and that the only thing holding the lead ballast keel to the hull was the
5200 applied as sealant.) The advice usually given about polyurethanes is:
if you think you might ever want to take the pieces apart again, use something

Weaker polyurethane

Life-Calk tube

Chemical companies have, however, been modifying this characteristic. For
example, 3M has created a new product known as 4200, which has nearly the
same strength characteristics as 5200 but lacks the same internal shear
strength, so two joined parts can be separated without the usual amount
of cutting, wedging, and swearing. Cure times for polyurethanes are relatively
slow (they cure completely in five to seven days and are tack-free in 48
hours), which can be a boon if you’re not quite sure about the fit of two
parts. As with polysulfides, though, curing times can be speeded up by misting
water on the exposed glue line.

There are other products that blur the distinctions between the families
of sealants and their properties, and they create even more havoc when you’re
trying to decipher the right product for the job. BoatLIFE introduced its
Life Seal several years ago. It is a blend of silicone and polyurethane
formulated specifically for use with fiberglass. It looks and handles like
silicone and comes in clear as well as the white and black of the silicone
family, but it has greater bond strength even when applied to wood and fiberglass.
It can be cut apart more readily than the polyurethanes, and it cures faster
than they do, too. It’s still not sandable or paintable, or ultimately as
flexible as silicones, but it is more so than the others and doesn’t have
the strong odor of the polysulfides. BoatLIFE says it’ll bond well to glass,
a difficult substrate to seal, and they recommend it for use either above
or below the waterline.

In addition to the 4200 and 5200 products, 3M also makes a 5200 with quicker
cure times. Called Fast-Cure 5200, it has most of the strength of regular
5200, but the cure time is significantly reduced: to tack-free in 1 hour
and fully cured in 24 hours – a real plus for manufacturers and others working
on small areas with lots of bedding. Like 5200, it can be cleaned up with
mineral spirits, though petroleum products won’t affect it once it’s fully
cured. At this writing, it comes in any color you want, as long as it’s
white, though the rumor mill suggests that black may on be the way.

Epoxy breakthrough

The Pettit Paint Company recently came out with a new epoxy-based
product that is redefining the use of that material in boat repair.
Called Flexbond Marine Epoxy, it (unlike other epoxy materials on
the market) retains a high elasticity component of about 40 percent
of volume. It bonds to fiberglass, wood, steel, and aluminum, above
or below the waterline, and after curing can be drilled and tapped,
sawn, planed, filed, screwed, or nailed without fracturing.

When it’s applied to overhead or vertical surfaces, it will not sag
or run. It cures under water. It doesn’t shrink or crack due to overheating
while curing, and it can be pre-tinted with all types of alkyd, polyurethane-,
or epoxy-based paints or tints without losing significant strength.

These attributes make it a fine repair product, much like any other
epoxy product on the market. What sets it apart is its flexibility.
Epoxies bond like there is no tomorrow to most other common substrates,
except for some plastics and glass. However, they are brittle and
the more of the proprietary fillers you use in the mix or the thicker
you apply them, the more brittle they become. Flexbond works the other
way. You use it as it comes out of the two-part tubes, without the
need to mix anything else into the matrix. You still get the bond
you need, but it is much better at flexing with the substrate as the
part moves with use.

This makes it a possible sealant in places where the greatest strength
in a bond line is necessary. The flexible joint that sometimes develops
between a ballast keel and the hull of a sailboat comes to mind, a
joint at which no other material seems to hold. Certain repairs on
wooden hulls could benefit, particularly with Flexbond’s ability to
hold a fastener. And it might be good for a quick-and-dirty repair

It’s too expensive to use just anywhere, and with a bond strength
even greater than that of the polyurethanes, you probably would want
to think about exactly how the part so joined was going to be used.
However, it seems to be a breakthrough in materials technology, one
which I think is going to spur a whole development of similar products
and one which will really expand our bag of boat repair tricks.

Sandable sealant

BoatLIFE also makes what the company calls a sandable sealant. They market
it as a product with which to pay the seams in teak decks. While polysulfides
have been the usual treatment for leaky deck seams, this silicone-based
product is a classic example of changes in sealant chemistry blurring the
lines of distinction between product lines. The main selling point is that,
unlike polysulfides, it fully cures in 24 hours and is tack-free and sandable
within 30 minutes, a real consideration on a project that is literally underfoot.
They say it has good-to-excellent bonding to most materials and recommend
it for sealing everything, in fact, except for wooden seams under water.
Like the other silicones, it is not paintable. It comes in the standard
deck-seam colors of black and white.

Deck seam sealants are obviously a large market, because 3M has also developed
a new product called Marine Teak and Wood Seam Sealant. A one-part polyurethane,
it’s an offshoot from the 4200 product. It differs from BoatLIFE’s seam
compound in that it takes a little longer to cure, though not as long as
the polysulfides. But, if need be, it can be painted other than its normal
black color by using lacquers. It might last a little longer than the silicone
but might be harder to remove later.

and Sealant Recommendations
selecting any caulks or sealants, refer to this chart and choose
the type that best suits your application.
– Excellent G – Good S – Satisfactory X – Not
to Fiberglass (Wood Trim)
to Wood (Wood Trim)
Seams ( Teaks and Other Woods)
Wooden Boat Hull Seams
to Hull Joints
Hull Fittings – Fiberglass
Hull Fittings – Wooden
& Lexan Plastics to Fiberglass
& Lexan Plastics to Wood
Hardware to Fiberglass
Hardware to Wood
to Wood (Deck & Hull Hardware)
to Fiberglass (Deck & Hull Hardware)
to Metal (Windshields)
to Fiberglass
to Wood
to Vinyl
Rails to Fiberglass
Rails to Wood
Shelf Life (Years)
Expectancy (Years)

What’s right for your job

Life Seal from BoatLife

What to use? It depends on what you’re going to do with it. Are you asking
it to hold back water and create a custom gasket, or do you want it to glue
two parts together? Are you doing deck seams or mounting a through-hull
fitting. A deck fill? An exhaust tip? How long can you leave it to cure
before the dog walks through it or you have to set sail and thrash it through
the ocean? And, like that ballast keel, is there a possibility that you
might someday need it to really hold?

Nine times out of 10, I find one of the general bedding products, like Life
Seal or 4200, works best. I mostly bed hardware to the hull or deck and,
having renovated a couple of boats, I’m very mindful of the fact that nothing
lasts forever. Some day somebody’s going to have to do maintenance on the
thing, and that next guy who has to tear it apart might just be me. As a
result, I don’t often have any use for something with the near-permanence
of 5200 or 292. Nor do I do much work on classic wooden hulls, so when I
buy Life Calk and 3M’s 101, I get it in the smallest tubes I can find.

I think that the most difficult decision would be what to use on those deck
seams, particularly since the two new alternatives seem to be such good
products. In general polysulfides in either one- or two-part mixtures, were
the best of the available choices, because until BoatLIFE’s development,
silicones weren’t recommended for wood, and the polyurethanes just took
too long (and the mess you could create with them was all but permanent).

The two-parts have
been recommended over the one-parts, as they cure faster and fill voids
better, though they can be a little “goopier” to work with. You
are somewhat limited by choice, as not all are available in both colors;
but all are advertised as having similar imperviousness to chemicals and
similar lifespans. One-part polysulfides, the silicones, and 3M’s product
all cost roughly the same, and the two-part polysulfide is the most expensive
of the lot. Perhaps the best idea would be to find someone with experience
in all of them, pick his brain and make your decision based on your own

Sealant selection

We asked several manufacturers why polysulfides are recommended for
“fiberglass” hulls and decks but not for some other plastics.

Two things can cause a manufacturer to not recommend a sealant for
use on a particular material: poor adhesion and chemical attack. Manufacturers
were much more concerned with poor adhesion than chemical attack.
They said they preferred to test and recommend on a case-by-case basis.
Naturally, selection charts are going to deal in general cases. So
follow the general selection chart, but double-check the container
and the manufacturer’s selection chart as well.

Some silicones are not intended for use below the waterline. Such
misapplication has caused boats to sink.

Apply it properly

The biggest complaint about any sealing job is, “I put two ounces
of stuff there, so how come it still leaks?” Invariably, when the
autopsy is performed and the part is removed, the sealant that is
there is microscopically thin and has failed because there wasn’t
enough of it to fill the space.

What happens is that the part installer becomes too gung-ho and tries
to do too much in one day. He’s applied the goop, installed the part,
cranked down hard on the fastenings, wiped off all the stuff that
leaked out around the edges, and gone on to the next piece. Well,
all that sealant is now on the rag, which will eventually end up in
the dumpster, not between the part and the hull where it will do him
some good.

Remember, that part is intended to be fastened to the hull for a good
long time and the space of a couple of hours isn’t going to be much
in the life span of the hull. Goop the part in a place where it will
have good contact with the substrate, install it lightly, and let
it sit a few hours for the sealant to cure. When you come back and
tighten the screws, there will be a nice, solid gasket under the part
that will compress and keep the water out. Keep the goop where it
belongs, under the part or in the tube. Boats are already too expensive
to be throwing away supplies because of misapplication.

Clean and prime

Above Below cleaner

With any of the products, remember: nothing is going to help them stick
to fresh-sawn, oily woods better than first cleaning the wood with products
such as BoatLIFE’s Life Calk Solvent and Cleaner and priming with their
Life Calk Primer or Sika’s 203 Primer. It’s false economy to try to do the
job and skip a step – it would be a shame to have to re-clean and re-pay
all those seams, either deck or hull, just to save the cost of the primer.
The old rule of thumb, When in doubt, prime it out, works just as well for
sealants as it does for paint.

For years, the biggest problem with all sealants has been the economy of
the packaging of the stuff. Fortunately, manufacturers have become aware
of the fact that, while a 10-ounce tube worked well for big jobs and boatbuilders,
most of the rest of us waste more than we use by having the stuff harden
in the tubes between uses. They’ve started selling most of their products
in one- and three-ounce toothpaste tubes that fit well in a toolbox and
contain just the right amount for the small jobs. They are also a lot easier
to handle in tight corners than caulking guns.

In the end, follow the manufacturers’ recommendations, be mindful of the
materials involved and the amount of bond strength you need, and don’t be
afraid to be liberal with the goop. More of it means less water in places
where you don’t want it.

For further information, contact:

BoatLIFE Industries 843-566-1225
3M Marine 877-366-2746
Pettit Paint Company 800-221-4466

Scott Thurston

Scott has returned three boatyard monsters to solid sailboat status
with the primary addition of elbow grease. He and his wife sail
their 1968 Camper-Nicholson 32, from Falmouth, Maine.

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