Understanding and Using Marine Sealants

Understanding and Using Marine Sealants
By Capt. Wayne Canning

Back when I started building and repairing boats things were fairly simple when it came to sealants. A couple of cans of Dolphinite seam and bedding compound, some cotton, a funny looking hammer and you were good to go. Then along came Silicone and Polysulfide. This was the beginning of new ways to keep water out of our boats. Today we are faced with a myriad of choices when it comes to sealants. So many choices in fact it has gotten difficult to figure out what is best to use and where to use it. To make matters worse, the more people you talk to about what sealant is best the more confusing it gets. With this in mind I wanted to compile a summary of what is available to most boaters and what the pros and cons of each are.
In the days of wooden boats, caulking was cotton or oakum (hemp fiber soaked in pine tar) driven into the seams and covered with a soft putty or tar pitch. Today’s caulks however, are sophisticated chemical compounds formulated for specific uses and conditions. Dispensed from tubes either by hand or with the use of dispensing guns they are not only easier to apply but offer superior performance and longer life. This performance though, is related to proper sealant selection for the job and proper preparation and use.
Today’s sealants have to perform in ever demanding roles. They have to be able to fill large voids and still cure fully, stick solidly to the substrate, and remain flexible through out there working life. Environmental conditions such as sunlight, temperature, and humidity all play a part in expected performance. Most modern sealants are single part, meaning they are air cured and not mixed. They are applied easy and have a long service life. That service life however can be adversely affected if they are not applied under the proper conditions and used with the correct substrates. Selecting the right product for the job at hand requires the user understand the properties and limitations of each type of sealant.
The following is a list of the common types of sealants the average mariner is likely to find available for use on their boat.
Silicones:
Silicones are one of the most common types of sealant available. First widely used in the early 60s they are a versatile and durable sealant. They are made by reducing silica (sand) into silicone polymer oil. Fillers such as Mica and Clay are then added along with other compounds to alter the characteristics and performance of the sealant. Silicone sealants adhere best to smooth, non porous surfaces such as glass, metal, fiberglass, and plastics. It will remain very flexible its entire service life, Silicone’s tear, cut, and abrasion strength is however, low. Silicones are self curing once exposed to air or more correctly the moisture in the air. One notable difference among the types of silicone sealants is the curing agent used. The three main types are; Acidic systems release acetic acid upon curing (you can tell these because they smell like vinegar); Neutral systems release alcohol during cure; and Aqueous systems release water during cure. Marine Silicones are supplied as a one part paste in both squeeze and gun tubes. Most Silicones are low sag and will not run or fall out of seams. They are available in many different colors. Most silicone sealants have very good resistance to chemicals, oils, and high
temperatures. Silicones also have very good UV and weathering resistance with useful exterior life span of 10-20years. They have a very good shelf life of several years and a fair open life of several weeks to months.
The smoother the surface the better the adhesion is with silicones. This combined with its excellent UV and weathering resistance make the non acidic cure types a good choice for bedding and caulking windows and ports and installing hatch lens. It works well with most plastics and is very good for bedding plastic and metal hardware to fiberglass surfaces. Keep in mind when using silicones that paint will not stick to it at all. This applies to the film left after clean up, care needs to be used to avoid getting any on any surface to be painted. Care should also be taken to select mould resistant Silicone sealants. Silicones contain acetates which feed mould; this can be a drawback when using it in a marine environment. This is a particular nuisance when using white pigmented silicon sealants as the mould shows up as back or gray spots. Silicones superior flexibility makes it a good choice for filling large gaps. Silicones are not recommended for underwater use on boats. Cure times are fairly predictable with the surface becoming tack free and skinned over in about ten minutes and stiff cure in about 45-90 minutes. Full cure takes about 24 hours. Cleanup is best done with denatured alcohol. Silicone sealants do tend to leave a thin film that often is not noticed until cured, thorough cleanup is important. Silicone’s can be applied in a wide range of temperatures above freezing but work best around 70 degrees.
Silicone Pros and Cons
Pros
Ease of use, Fast cure, Long life, Good Flexibility over its life, Comes in many colors including clear, Inexpensive, Non Toxic once cured, Excellent UV and weathering resistance, Good resistance to chemicals, oils, and high temperatures, Long package life even once opened, Low shrinkage rate, Large gap filling, Non acidic type good for sealing electrical connections, Can be applied in a wide range of temperatures.
Cons
Cannot be used underwater, Paint will not adhere to it, Will support mould and mildew growth, Short tack free times mean you must work fast on larger projects, Low tear strength, Poor abrasion resistance, Harder cleanup (leaves film) does not sand well.

Polysulfides:
Polysulfide polymers first came into practical use during WWII to help patch bullet riddled aircraft. It was introduced as commercial a sealant in early 1950s. Polysulfide sealants are formulated using a base polymer, curing agents, reinforcing fillers, plasticizers, and adhesion additives. Formulations vary from company to company with variations in viscosities and cure properties. Polysulfide’s are available as single part air cure or as a two part mix. The common air cure types have a average tack free time of 5-24 hours and full cure times of 5-20 days. Polysulfide’s cure to a firm rubber like texture that adhere well to a variety of substrates including glass, wood, metals, and fiberglass. They have good tear, cut and abrasion resistance. Polysulfide’s are supplied as a one part paste in both squeeze and gun tubes with low sag and run or fall out rates. Polysulfide’s are available in limited colors mostly white, black, brown, and gray. They have good resistance to chemicals, and oils, but poor resistance to high temperatures. Polysulfide’s also have fair to good UV and weathering resistance with a useful exterior life of 10-20 years. They have a good unopened shelf life of several years and an open shelf life of several days to 2-3 weeks. They are best applied at temperatures near 70°. Low humidity levels will slow cure times.
Polysulfide’s can be used above and below the water line making them a good choice for sealing underwater fittings and thru hulls. They are not recommended for use with most plastics used for marine hardware, hatches, and portlights. The reason for this is that most common plastics contain oils that make it difficult for the polysulfide sealants to adhere to them. Polysulfide’s can be sanded and painted making them a good choice when working around painted surfaces. If used on woods particularly oily woods such as teak it is best to use a primer first as recommended by the sealants manufacture. Due to slower cure rates care needs to be taken with environmental conditions during the full cure time frame. Polysulfide’s contain few acids and are a good choice for chain plates and sealing around rigging wire. The weathering and UV resistance characteristics are not as good as the silicones. Polysulfide sealants are stronger than silicones however they are not as flexible, this should be kept in mind when filling larger gaps. Polysulfide’s resistance to oils makes it a good choice for filling joints in teak decks, its ability to hold paint make it a good choice for sealing joints on painted wood. Polysulfides are also available in low viscosity types that are useful for filling small cracks. Application temperatures and humidity affect the cure rate and overall performance of the product. Polysulfide’s do not do well in high temperatures, this could include metal deck fittings, that when heated by the sun can get surprisingly hot. Cleanup can be done with denatured alcohol or mineral spirits.
Polysulfide Pros and Cons
Pros
Ease of use, Good working times, Long life, Good Flexibility over its life, Can be used underwater, Easy Cleanup, Fair to good UV and weathering resistance, Good resistance to chemicals, oils, Good shelf life even once opened, Low shrinkage rate, Paintable, can be sanded, Will not support mould and fungus growth, Higher tear and cut, and abrasion resistance than silicone.
Cons
More expensive than Silicones, Cannot be used with many plastics, Poor resistance to high temperatures, Gap filling not as good as silicones, Limited color selection.

Polyurethanes:
First manufactured in the early 70’s Polyurethanes contain calcium carbonate and/or titanium dioxide and different types of urethane polymer. They are manufactured as single part air cure and two part catalyzed systems. Formulations vary from company to company with different viscosities, cure properties, and performance characteristics. The Polyurethanes are as much an adhesive as they are a sealant. The most well know of these is the 3M product 5200. They adhere aggressively to many surfaces, cure to a hard rubber like material with extremely good tear cut and abrasion resistance but are less pliable than the silicones and polysulfides. Most are supplied as a single part, air cured product. Dispensed from squeeze and gun tubes they have a thick paste like consistency when uncured with good sag and drop out resistance. Most become tack free in 48 hours and fully cure in 5-7 days. Fast cure formulations are available that have tack free times of 1-2 hours and full cure in 24 hours. They come in limited colors of white, black, tan, and brown. The Polyurethanes have good chemical resistance but poor oil and heat resistance. They have fair to good UV and weathering resistance but can suffer from age hardening. Prolonged submersion can chemically alter the cured product reducing its strength somewhat. Useful working life can be as much 20 years. Self life is limited to about 1 year and open life a few days. They are best applied at temperatures near 70°.
Due to their great tear strength and aggressive adhesion, polyurethanes are best used for applications that are under high loads. Disassembly, once cured, can be difficult and will often damage the substrate. For this reason it is wise to only use it on assemblies that are not expected to be disassembled such as hull deck joints. Many companies are now producing formulations that are not as aggressively adhesive and have lower tear strength making them more practical for bedding hardware. Polyurethanes have fair gap filling characteristics, can be painted and are very compatible with urethane paints however some single part paints may not cure properly when applied over sealant. They are very useful for making “beauty beads” or cosmetic fills prior to painting. Polyurethanes should not be used where oil exposure is possible. This includes natural oils found in teak and teak deck oils. It is not recommended for most plastics as it will not adhere well due to oils in the plastics. Although it can be used on glass, primers are recommended as peeling has been noted due to UV degradation. Cleanup can be done with mineral spirits.
Polyurethane Pros and Cons
Pros
Ease of use, Good working times, Long life, Can be used underwater, Easy Cleanup, Fair UV and weathering resistance, Good resistance to chemicals, Low shrinkage rate, Paintable and can be sanded, Will not support mould and fungus growth, Highest tear strength of all the sealants, Good abrasion, and cut resistance.
Cons
More expensive than Silicones but not as expensive as the polysulfide’s, Cannot be used with many plastics, Poor resistance to high temperatures, Often too aggressive of a bond making disassemble difficult, Poor oil resistance, Poor shelf life and very poor open container life, Some paints will not cure properly when applied over sealant.
Polyether sealants:
One of the newer sealants to hit the market polyether’s are a hybrid of urethanes and silicone. These polymers are also known as modified silicones, combining the chemistry of urethanes and silicones. Polyether is a so called green sealant in that they release no VOC’s during cure times; this also means they have no shrinkage. Because they are a hybrid they have the good adhesion characteristics the urethanes and the flexibility, UV, and weather resistance of the silicones. Polyether’s are single part air cured with tack free times of 1-2 hours and full cure times of around 48 hours. Fast cure versions are available with full cures times of around 24 hours. Dispensed from squeeze and gun tubes they are a thick past with low sag or drop out. As of this writing colors are limited to white and black. The Polyether’s have good resistance to oils and chemicals and are recommended for use above and below the waterline. They have very good UV and weathering resistance. They are reported to have a working life of 20 years. Self life is several years and open life several weeks. They are best applied at temperatures near 70°.
Polyether’s are a good general purpose sealant that can be used above and below the waterline. The cured sealant can be sanded and painted. It is good for most materials such as glass, wood, metals, and some plastics. As these are Hybrids and formulated differently users should check labels carefully before using with plastics and some paints. Polyether’s are a good substitute for silicones when bedding most deck hardware with better adhesion than silicones yet retain the excellent UV and weather resistance .

Polyether Pros and Cons
Pros
Ease of use, good working times, Long life, Good Flexibility over its life, Can be used underwater, Easy Cleanup, Excitant UV and weathering resistance, Good resistance to chemicals, Low shrinkage rate, Paintable, can be sanded, Good tear strength, Good abrasion resistance.
Cons
Most expensive of all the sealants, Not all brands can be used with plastics and some paints.
Other Hybrids
There are other hybrids on the market as well. Silicones blend well with other materials making them a good choice for hybrid blends to improve the weathering resistance of the base product. Some types of hybrids are specially formulated for specific projects such as teak decking or window sealing. Generally they are more expensive but can be worth the expense if used correctly. As many of these products are somewhat new it would be advised to buy a small tube and do some test work before using them on a big project.
Butyl Sealants
Butyl Sealant is formulated from a blend of butyl rubber and polyisobutylene to form an economical, flexible sealant. It is used widely as a hot melt sealant, preformed dry sealing tape or gasket, and as a gun-dispensable sealant. Butyl sealants are normally low strength and tend to creep under load. They tend to be very tacky and will stick on contact. Most butyl based sealants are non-curing or slow curing and not recommended for large joint movement applications. Butyl’s have good chemical, oil, and UV resistance but poor weathering resistance. Butyls are available in limited colors with the most common being black. Useful life span varies but is generally around 10-20 years.
Used primarily within the building and construction industries it has some limited use on boats. Butyl tapes are very useful for setting glass into frames creating a long lasting seal as long as primers are used. It also has some limited specialized uses in tape form for making gaskets. Some sailors have had good results using butyl tape around chain plates. Cleanup is best done with acetone.

Sealant Failures
Before trying to figure what sealant is best for any particular application it helps to understand why sealants fail in the first place. There are three types of sealant failure: adhesive, cohesive, and substrate. Simply put, the bond between the sealant and the substrate can fail, the sealant itself can fail or the substrate can break. That said most problems with caulked joints are commonly due to one of two errors. Either the substrate was not effectively prepared, or the wrong product was selected. I often hear complaints of sealants failing a short time after application. Often the sealant its self is mistakenly blamed for the failure, the user swearing they will never use that sealant again. The problem more often than not is that the wrong sealant was selected for the task at hand. A classic example of this is the use of polysulfide or polyurethanes being used for installing plastic portlights. These sealants, do not stick to these types of plastics due to oils in the plastics. It is not a failure of the sealant but a failure of the installer to select the right product for the task. Once you have selected the correct product care needs to be taken to use it correctly and not over extend its abilities. Filling seams too large or with too much movement will lead to failure. Sealants also need to be allowed to expand and contract within the seam. Sealants pushed into a 3 sided channel are restricted in full movement and will eventually crack and fail. Substrate failures are often seen when caulking over painted surfaces. The paint will fail and lift off the surface allowing water to enter under the paint not the sealant. Other types of failure can relate to age, weathering, sun damage, mould and mildew damage. Some sealants can be softened or damaged due to the use of chemicals, oils and solvents while maintaining the boat. While most sealants have good chemical resistance when cured, the use of chemicals during clean up or when smoothing down the wet sealant can adversely affect their chemistry and life span as well.
Preparation
Once the proper sealant has been selected for the job at hand, proper preparation is the next key to success. For re-caulking an existing installation, the first task will be to remove the fitting. If the fitting is put down with polyurethane it will be hard to take up. It is recommended to tape around the fitting to protect the surrounding area. Once the fasteners are out, start by slowly working a thin screwdriver under the fitting while cutting the caulk with a razor knife. Anti Bond 2015, a solvent and softening agent is very useful for helping remove Polysulfides and polyurethanes. Work some Anti Bond under the fitting as you go. Take your time and do not try to lift the fitting faster than it wants to go. Stopping from time to time to, while leaving a wedge under the fitting will give the sealant time to “let go.” It is best to start on one side and peel up to the other. A thin serrated knife will also help to cut the sealant. Once the fitting has been removed be sure to completely clean off any old sealant and residue. A sharp chisel will help remove the old sealant as close to the surface as possible. Pulling the chisel backwards like a furniture scraper is also a good way to remove stubborn thin sealant remains. Solvents and cleaners can help remove the final film. Scrubbing with an abrasive pad will also help remove the film. A final light sanding with 220 grit paper will leave a surface with some “tooth” for the new sealant will help. If sealing to fiberglass wipe down with Acetone to remove all traces of mold release wax. Even on older boats the wax can remain for many years if covered. Make sure any solvents have completely evaporated prior to applying the new sealant. If removing a fitting sealed with silicone it is best to reseal with silicone as it is almost impossible to remove all traces of the silicone. Silicone will stick to itself but other materials will not stick to silicone so it is best to stick with silicone.There are basically three types of caulking. Seam filling, bedding of parts, and “beauty beads” or fillet beads. Proper preparation of each will ensure success.
When seam filling it is important to make sure the seam to be filled is not too deep or wide for the sealant. Sealant applied too thickly may not cure properly this particularly true of the air cure sealants. To ensure long life of the sealant it is important to have the sealant sticking to only two sides of the seam. This means it should not be allowed to adhere to the bottom of the seam. If it does the sealant will not be able to fully flex with the seam expansion and contraction. This will lead to cracking and ultimately failure of the sealant. If the seam is wide and deep I recommend putting neoprene weather stripping or rod into the seam as a filler then applying the sealant over that. This will reduce the mass of sealant needed and prevent any bottom sticking. If the seam is small and shallow like the seams in teak decking a thin strip of masking tape called a “bond breaker” is put in the bottom of the seam prior to applying the sealant. Some sealant manufactures recommend a primer be applied prior to the substrate. Check with the sealant manufacture before starting.
Bedding parts and fittings is relatively easy, but there are a few tricks that will help improve success. Drill all holes needed and dry fit the fitting before getting the caulk out. It is recommended to counter sink the screw or bolt holes under the fitting or better yet on the fitting itself. This will leave a channel for the caulk to form a gasket around the fastener. Slipping a small O ring around bolts and into the counter sink area to create an additional seal around each fastener will also ensure a good seal. Always clean new parts with solvent prior to installing as many new parts have tooling oils or other chemicals left of their surface from manufacturing. Use caution when using solvents on plastic and if in doubt test a small area first. A light sanding will also help give the surface some tooth for the sealant to stick to.
Fillet caulking, normally done in corners, requires little prep other than making sure the area is clean and free of contaminates. If a crack is present try to open it up a bit with a scraper to give an opening for the sealant to get into. It can help to cut a bit of a V into the seam to give the sealant more to “hang on to.” If the surface has been painted check for loose paint and repair prior to applying the sealant. Flush with a solvent to remove contaminates. Make sure the seam is completely dry.
To tape or not to tape? This can lead to some heated discussions around the boat yard, as some swear by it and others do not. Having done both I prefer not taping myself but everyone has their preference. The only time I tape is when working with hatch lens and sometimes when installing black framed portlights using black sealant. If masking tape is used, remove it while the sealant is still wet, peeling back and away from the seam at a low angle. This will prevent the tape from lifting the sealant from the edge of the seam. Taping takes additional time and can be very tedious, particularly so when doing teak decks. In most cases if the sealant is applied correctly there will be no need for tape. On the other hand some may find the tape worth the extra time with the time savings in clean up.
Applying the sealant
The sealant has been selected, the project prepped so now it’s time to get the caulk gun out and get started right? Not so fast, before the tube is opened we need to talk about cleanup. Most sealants become tack free fairly quickly, (meaning the surface skins over) even more so when working out in the sun and wind. You have to have all your cleanup supplies ready to go before you start so you can clean up as you go. It is also a good idea to have a clear idea of how to do the cleanup. If doing teak decks or other wood caulking cleanup is easy, just leave the sealant till it has cured and sand it down flush. Most sealants shrink while curing; leaving the bead high in the seam will ensure it will be flush when finished. This also reduces stress on the sides of the seams during cure. Pretty much any other project will require you clean up as you go. Small projects like bedding a fitting are easy just use a putty knife to remove the excess and wipe the film up with a solvent soaked rag. Trim the wet caulk back as close to the fitting as possible with the putty knife. Use a narrow tool for getting into tight places. Remove as much excess as possible so that it does not smear and spread as it is wiped it down. When cleaning excess sealants always use solvents recommended by the sealant manufacture. Non compatible solvents used for cleanup can affect the cured properties of the sealant. If excess sealant needs to be removed to form a filet you can use a plastic putty spreader that has been cut into a radius the size needed for the fillet. This will make a very clean concave surface. The rounded spreader will push the excess sealant off to the side where it can be scooped up with a putty knife. If the surface of the sealant needs further smoothing, lightly spray with soapy water then smooth as needed. Never use solvents for this. Have plenty of clean rags available and a trash can to put soiled rags into. It can be surprising the mess a rag with wet sealant will make when the wind blows it down the deck.
Now it is time to start caulking. Cut the tip off of the tube at about 30°-45°. Puncture the inner seal with a nail or other tool. Push some sealant up the nozzle till it just starts to come out. Some sealants will have some air in the cartridges. This will get compressed when you push down on the plunger causing the sealant to keep coming out after the trigger has been let go. It is a good idea to release the pressure on the plunger when you finish running a bead. Doing this will save sealant and reduce the mess. I always set the tip over a rag when not using it, and wipe the tip clean before starting another bead. Another big controversy with caulking is whether it is better to push or pull as you run your bead. Pulling the gun towards the user allows you to see the sealant bulge up and overflow the seam as you go. This can be done by pushing as well so it really comes down to personal preference. No matter what method you use it is important to see the sealant go fully into the seam and overflow just a bit. Avoiding trapped air pockets is important to a leak free seal. Always apply more sealant than needed then clean up the excess, this will ensure the sealant is fully doing its job without voids. When bedding through bolted parts be careful not to turn the bolts but rather only tighten the nuts. This will prevent the sealant from being pulled out of the bolt hole by the treads on the bolts. If the holes are counter sunk as suggested earlier the sealant will fill the counter sinks and make a good seal even if most of the rest of the sealant is squeezed out from under the fitting. I have heard some people suggest not fully tightening the bolts till the sealant is full cured. The thinking here being that you will not squeeze all the sealant out from under the fitting. The problem with this is it is all too easy to let the bolt turn when doing the final tightening. Even a small turn will break the seal around the bolt.
As can be seen not all sealants are good for all jobs. Unfortunately there is no one sealant that will work for all things. Silicones and Polyether’s are the most useful for general purpose work. The polyurethanes are only really useful for high load applications and the Polysulfide’s are best for underwater use. Understanding a sealants limitations and strong points is important to picking the right product for the right application. Careful selection is the first step towards a successful job. Once the proper sealant is picked, careful preparation and application will result in a clean professional look and a long lasting seal.

Wayne Canning lives on his Irwin 40 Vayu, in Wilmington NC. A marine professional for more than 35 years he now is a full time Marine Surveyor, runs a web site for other professional marine surveyors and a site for those restoring project boats. He also, provides services as project manager for boat restorations. Visit www.projectboat.info, or www.4ABetterBoat.com for more information.