These are notes for a guest lecture I made at a boat building school in October 2018.
Content Warning: The following blog post contains highly technical material that may be harmful, or just plain boring, to some readers...
You only need a few materials in your boatbuilding toolbox:
- Polysulfide and silicone -- both are sealants (they are NOT adhesives)
- Epoxy -- adhesive
- Polyurethane -- “3M 5200” -- sealant and adhesive
- And maybe butyl rubber tape for bedding deck fitting
All sealants are “one part” -- they are ready to use, no mixing.
Epoxy is “two part” -- resin + hardener, plus a filler or additive -- “customizable” for specific application and conditions.
There is no boatbuilding adhesive or sealant that is both stiff/rigid (that is, not soft and flexible) AND is removable -- the closest thing might be butyl rubber tape…
Polysulfide Sealant
- Thiokol-based polysulfide sealant. The first ‘high performance’ sealant chemistry
- Excellent adhesion, UV resistance, and chemical resistance.
- Use ABOVE or BELOW the waterline with wood, metal, fiberglass, polyethylene.
- Do not use on thermoplastic material like polycarbonates and acrylics.
- PROS: Remains flexible forever. Resistant to everything but abrasion. Excellent for bedding if you will ever need to take the fitting apart. Unlike silicone sealer, it can be used as an adhesive, with limited strength. Adheres to oily woods such as teak, resistant to oils and sealers. Long working time, and long shelf life, even after being opened.
- CONS: More expensive than silicone (but the shelf life after being opened is much longer). Even the high viscosity version will flow and can be messy. Has a long cure time -- 2 to 7 days; and is getting harder to find.
Silicone Sealant
- RTV - “room temperature vulcanization” -- moisture curing - smells like vinegar (acetic acid).
- Compatible with all plastics, metal, glass, fiberglass. High-temp (red) silicone is good to 650 degrees F.
- NOT AN ADHESIVE -- use only above the waterline. Can’t be painted.
- Maintains high flexibility indefinitely; excellent for isolating dissimilar metals.
- PROS: Highly resistant to chemicals and UV. Insensitive to temperature. Doesn’t burn easily.
- CONS: Most expensive sealant; has a short shelf life once opened and exposed to moisture.
Polyurethane Adhesive-Sealant -- “5200”
- Fast setting, single-part, moisture-curing adhesive/sealant which bonds with enormous strength, excellent long-term UV resistance, and superior flex in stressed joints.
- Use ABOVE or BELOW the waterline with metal, wood, foam, glass. Good for hull/deck joints and bonding thru-hull fittings but incompatible with ABS and Lexan.
- Do not use with rubber or plastic materials.
- DO NOT USE a polyurethane adhesive if the bonded parts might ever need to be separated at any time in the future.
- PROS: Creates a permanent bond. Gap filling, but will flow, so tight joints are needed. Cheaper than silicone, polysulfide and polyether adhesive-sealers -- if used in quantity.
- CONS: Very short shelf life once opened; expensive if used in small volumes. High flow and sag. Extremely messy. More sensitive to surface condition than polysulfide sealant. Long cure time (2 to 7 days). Might attack plastics. Contains isocyanates. Avoid skin and eye contact. The use of protective gloves is highly recommended.
Epoxy
- 2-part thermosetting adhesive and coating; 100% solids. Completely waterproof when used correctly. Tolerant of surface and application conditions. And excellent gap filling.
- Use ABOVE and BELOW the waterline with wood, abraded or passivated metal, fiberglass. Will bond to “hard” plastics and old epoxy. Use as an adhesive, a coating, or a sealant.
- Do not use for joints that flex or thermally expand/contract significantly. Does not readily bond to StarBoard or polyethylene (“milk bottle”) plastics. BUT you might be able to use one of the advanced “flexible” epoxy to bond those.
- PROS: Lower cost per pound than any other adhesive or sealant. Multi-use: the same resin-hardener can be used with various fillers to modify strength and viscosity. Tolerant of use conditions. Fast cure (overnight). Infinite shelf life; and easy to store once you open the container. Many competitive suppliers, and lots of user “how to” info.
- CONS: Long-term UV degradation. Sensitization due to exposure to components and sanding dust. Short working time, and sensitive to ambient temperature and humidity.
SUMMARY
- Don’t use polysulfide for plastic fittings (some); or if there large movement or vibration.
- When you use silicone, if you ever want to paint or epoxy to that surface you will need to physically remove the layer with any silicon contact (sand or solvent).
- Don’t use “5200” for bedding if you ever want to get the fitting apart in the future.
- Epoxy (with one exception -- see below) has to be protected from UV. It WILL deteriorate if it is exposed to direct sunlight for long enough time.
EPOXY TIPS...
- Use epoxy safely -- wear gloves, eye protection, and work outside if you can. Wear the right respirator if you are working in an enclosed space.
- Don’t mix different brands of epoxy resin and hardener -- the resin and hardeners are formulated to work together, and mixing brands may yield weak joints.
- Choose the right hardener for the climate, temperature, and application -- SOME, BUT NOT ALL, hardeners have UV inhibitors. Check and TEST. Or plan on painting or varnishing.
- Follow the mixing volumes -- use pumps OR scales, and make sure you check the mixing ratios if you change suppliers. Too much hardener is worse than too little -- too much will “plastisize” (or soften) the cured epoxy (see Epoxyworks, No. 47, Fall 2018, page 20).
- Use the correct filler, viscosity modifier, and additive for the job.
- Try to work so that you don’t have to sand, or at least minimize sanding. If you are using multiple coats (e.g., to fill glass fabric), recoat when the first coat is tacky. Use a rasp (four-in-hand or Stanley Surform). The Shinto Saw Rasp (from Duckworks Boat Builders Supply) works very well and gives you a rough and a fine surface in one tool. And they stay sharp forever.
- For encapsulation, thin the first coat with isopropyl alcohol, and overcoat with one or more layers of full strength epoxy as soon as the “primer” coat is tacky. You can also clean up drips with alcohol.
FILLERS and MODIFIERS are used to adjust the viscosity of the mix and to increase the strength or sandability of the cured epoxy
- Wood dust -- to match wood - maybe. Cheap and good for forming fillet joints that will be covered with glass. Not good for surface joining.
- Powdered limestone -- another cheap filler, available at any hardware store or lumber yard.
- Silica (“colloidal silica”) -- for the strongest joints and fillets without adding glass cloth.
- Microballons -- for fairing/smoothing a surface, to fill low areas and voids with an easily sanded material.
ADDITIVES
- Graphite powder -- good for abrasion resistance and low resistance dry bearing surfaces.
- Aluminum powder -- increase hardness, abrasion and moisture resistance.
- Colorants -- white, black, grey (West Systems). Red, yellow, blue, green, brown (System Three).
REINFORCING MATERIALS are used to increase strength or abrasion resistance
- Glass tape -- to reinforce joints (most common is 9 ounce).
- Glass fabric -- for stiffening thin plywood panels and for increasing abrasion resistance.
- Biaxial tapes and fabrics -- weave crossed at 45 degrees; 12-ounce; 24-ounce tape.
The cost (about $1/oz, by weight) and density of glass cloth + epoxy is much greater than plywood.
To “build light”, minimize the amount of glass that you use: thinner (that is, lower “ounce”) glass. Or, try epoxy/glass on one side and epoxy coat (no glass) the opposite side.
4- and 6-oz glass will be almost invisible when wet out.
*wet out glass plus two fill coats (from West System). Density of epoxy is 1.18 gm/cc (water is 1 gm/cc) or 9.85 pounds/gallon (1.23 oz/fluid ounce)
Weight- and cost-adding example:
- A sheet of 6 mm okume plywood weighs about 21 pounds...cost $65.
- A 6-oz glass layer will add about 6 pounds to a sheet of plywood…and will require 2 quarts of epoxy (about $60-70).
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Adhesives and Sealants for Boatbuilding -- HANDOUT
You only need a few materials in your toolbox:
- Polysulfide and silicone -- both sealants (are NOT adhesives)
- Epoxy -- adhesive
- Polyurethane -- “3M 5200” -- sealant and adhesive
- And maybe butyl rubber tape for bedding deck fitting
*wet out glass plus two fill coats (from West System)
Epoxy Material Sources
- Duckworks Supply -- http://www.duckworksbbs.com/
- Jamestown Distributors -- https://www.jamestowndistributors.com
- Fiberglass Supply -- http://www.fiberglasssupply.com/
Info Sources
- “The Epoxy Book,“ System Three Resins, https://www.systemthree.com - FREE.
- “Epoxyworks,” twenty-five years of articles, technical data, and tips from the WEST SYSTEM folks: http://epoxyworks.com/index.php/back-issues-index/.
- “The Gougeon Brothers on Boat Construction,” 5th Edition, Meade Gougeon. The original epoxy bible. Lots of technical info and techniques. If you read just one book, download at: http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf .
- “Epoxy Basics,” Russell Brown, hard copy and e-book from the Port Townsend Boatworks, available at http://www.duckworksbbs.com/media/books/ptw/index.htm .
- “Small Craft Advisor” magazine, series of articles by Kern Hendricks of System Three Resins, starting in Issue No. 109 (Jan/Feb 2018).
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