Sealants, Adhesives, and Lubricants
Choosing the wrong sealant is worse than using no sealant at all — the right product in the right joint is the difference between a dry boat and a chronic leak.
Polyurethane Sealants — 3M 4200, 5200, and Sikaflex
Polyurethane sealants are the workhorses of marine bedding, and the differences between them are critical to understand because choosing the wrong one creates either a joint that leaks or a joint that can never be disassembled. 3M 5200 is a permanent structural adhesive sealant. It bonds with extraordinary tenacity to fiberglass, wood, aluminum, and most other substrates. Once cured, it is virtually impossible to remove without destroying the substrates — attempting to remove a deck fitting bedded with 5200 typically means grinding, chiseling, and accepting damage to the gelcoat and possibly the laminate beneath. Use 5200 only where you want a permanent bond: hull-to-deck joints, keel-to-hull joints, and structural members that will never need to be removed.
3M 4200 is the removable alternative — a polyurethane sealant with adhesion properties strong enough to seal but not so strong that disassembly is impossible. Fittings bedded with 4200 can be removed with patience, a thin putty knife, and some dental floss or Dyneema line worked under the fitting to cut through the sealant. This is the correct choice for deck hardware, cleats, stanchion bases, portlights, and any fitting that may need to be removed for rebedding, maintenance, or replacement. The number of boats with hardware bedded in 5200 that shouldn't be is staggering — and the owners of those boats discover the mistake when they need to replace a leaking portlight and realize they'll have to destroy the frame and the surrounding gelcoat to get it off.
Sikaflex 291 (general purpose) and 295 UV (for direct-glazed windows) are the European equivalents that compete directly with the 3M products. Sikaflex 291 has adhesion properties between 4200 and 5200 — stronger than 4200 but not as aggressive as 5200. Many professional boatbuilders prefer Sikaflex 291 for deck hardware because it provides excellent sealing and enough adhesion to keep a fitting from shifting, while still being removable with mechanical effort. Sikaflex 295 UV is specifically formulated for bonding glass and polycarbonate panels directly to frames — it has the UV resistance that standard polyurethanes lack, preventing yellowing and degradation at the bond line.
All polyurethane sealants share the same surface preparation requirements, and skipping prep is the number one cause of sealant failure. Both surfaces must be clean, dry, and free of old sealant, wax, mold release, and contamination. Clean with the manufacturer's recommended solvent — 3M Adhesive Cleaner or Sika Aktivator 205 — not just acetone, which can leave residues on some substrates. Some substrates (teak, certain plastics, aluminum) require a primer for reliable adhesion — Sika Primer 209D for porous substrates, Sika Primer 210T for metals and plastics. Skipping the primer on teak decks is a guaranteed sealant failure — the oils in teak prevent direct adhesion of polyurethane sealants.
Application technique matters as much as product selection. Apply sealant in a continuous bead, set the fitting, and snug the fasteners just enough to achieve light squeeze-out around the entire perimeter. Do not overtighten — compressing the sealant to a paper-thin film defeats its purpose. The sealant needs to maintain a thickness of at least 1/16 inch (1.5mm) to function as a flexible gasket that accommodates thermal expansion and structural movement. Once squeeze-out appears all around, leave the fasteners at that tension and allow the sealant to cure for the manufacturer's recommended time before final tightening — typically 24-48 hours for polyurethanes.
Label every fitting on your boat with the sealant used to bed it. A small piece of engraved tape or a note in your maintenance log saves enormous frustration when it's time to rebed. If you don't know what sealant the previous owner used, assume 5200 and plan accordingly — heat guns, solvents (DeBond, 3M adhesive remover), and patience.
Never use 3M 5200 to bed deck hardware that may ever need removal. This is the single most common sealant mistake in the marine world. The cost of bedding a stanchion base in 5200 instead of 4200 is approximately zero at installation and approximately $500-$1,000 in gelcoat damage and labor when you need to remove it. The time to make this decision is before you squeeze the trigger on the caulking gun.
Polysulfide Sealants, Butyl Tape, and Life Calk
Polysulfide sealants (Boat Life Life Calk, 3M 101) were the standard marine sealant before polyurethanes took over, and they still have specific applications where they outperform polyurethane. Polysulfides are inherently flexible, have excellent chemical resistance, and maintain their seal over decades — many boats from the 1970s and 1980s still have original polysulfide bedding that remains watertight. Their key advantage is that polysulfides remain permanently flexible and never achieve the aggressive adhesion of polyurethane, making them ideal for joints that need to accommodate significant movement.
Life Calk (polysulfide) is the go-to sealant for wood-to-wood joints on traditional boats — plank seams, garboard seams, and cabin trunk joints. Unlike polyurethane, polysulfide does not bond aggressively to wood, which is important because wood moves constantly with moisture cycling. A polysulfide seam flexes with the wood; a polyurethane seam either holds the wood rigid (creating stress fractures) or breaks free and leaks. Life Calk also works well for bedding portlights in aluminum frames, where the differential thermal expansion between aluminum and fiberglass requires a sealant that can accommodate movement without breaking its seal.
Butyl tape (butyl rubber sealant tape) is the unsung hero of marine bedding, and in many applications it is the best choice for reasons that have nothing to do with adhesion strength. Butyl tape — sold as Bed-It Tape, MainSail butyl tape, or in bulk rolls from commercial suppliers — is a non-curing, permanently sticky, completely waterproof tape that beds deck hardware without any chemical bond to the substrate. It relies on compression from the fasteners to maintain its seal. The advantages are significant: butyl tape never cures, so removing a fitting bedded with butyl is as easy as backing out the screws and lifting — no solvents, no chiseling, no gelcoat damage. It conforms perfectly to irregular surfaces, fills gaps, and maintains its seal for years.
Butyl tape is the correct choice for bedding deck hardware on cored decks — cleats, stanchion bases, genoa tracks, and similar fittings. The reason is practical: cored decks require periodic inspection and occasional rebedding as the core compresses under fastener loads over time. A fitting bedded in butyl can be removed, the core checked, the fitting rebedded with fresh butyl tape, and the job is done in 30 minutes. The same fitting bedded in 4200 takes hours of careful removal, solvent cleaning, and reapplication. Professional riggers and experienced cruisers increasingly use butyl tape for virtually all deck hardware — it seals just as well as polyurethane and is infinitely easier to service.
The critical limitation of butyl tape is that it has no adhesive strength — it seals but does not bond. It relies entirely on fastener compression to maintain contact with the surfaces. This means it is not appropriate for applications where the sealant must also serve as an adhesive — hull-to-deck joints, structural bonds, or any joint where the sealant must resist tension or shear loads. For those applications, you need polyurethane or structural epoxy.
Tools & Materials
- Butyl tape (Bed-It Tape or equivalent), 1/8" x 3/4" roll
- 3M 4200 Fast Cure polyurethane sealant
- Sikaflex 291 general purpose sealant
- Life Calk polysulfide sealant (for wood applications)
- Plastic scraper for squeeze-out removal
- Sika Aktivator 205 or 3M Adhesive Cleaner
- Sika Primer 209D (for porous substrates)
When bedding with butyl tape, wrap the tape around the base of the fitting so it covers the entire footprint and extends slightly beyond the edges. Set the fitting, run the fasteners down until you see slight squeeze-out all the way around, then trim the excess with a plastic scraper. The squeeze-out confirms you have full compression with no dry spots in the seal.
Structural Adhesives — Epoxy and Methacrylate
When you need a joint that carries structural loads — bonding a bulkhead to a hull, tabbing structural members, gluing core material to skins, or repairing fractured laminate — you're in the domain of structural adhesives, and the dominant player is marine epoxy. WEST System (specifically the 105 resin with 205 fast or 206 slow hardener) has been the industry standard for decades. TotalBoat and MAS Epoxies offer comparable products at somewhat lower prices. System Three and Pro-Set are also professional-grade options. These are not interchangeable with hardware store epoxy — marine structural epoxies are formulated for controlled mix ratios, predictable cure profiles, and reliable mechanical properties that hardware store 5-minute epoxy cannot match.
WEST System 105/205 (fast hardener) and 105/206 (slow hardener) differ in working time and cure temperature requirements. The 205 fast hardener gives a pot life of approximately 9-12 minutes at 72°F and is the choice for small repairs and bonding operations where you can work quickly. The 206 slow hardener extends the pot life to 20-25 minutes and is essential for large lamination jobs, wet-out of big pieces of cloth, and any operation in warm conditions where fast hardener would begin to gel before you finish the layup. In temperatures below 60°F, both hardeners slow dramatically, and below 50°F, the cure may be incomplete — you'll get a soft, under-cured laminate that never reaches full strength.
Thickening agents transform liquid epoxy into gap-filling adhesive or fairing compound. Colloidal silica (WEST System 406 or Cab-O-Sil) mixed into epoxy creates a strong, non-sagging structural adhesive for bonding operations — this is what you use for tabbing bulkheads, bonding stringers, and structural fillet joints. The consistency should be like smooth peanut butter — thick enough to hold a vertical fillet without sagging but fluid enough to wet the surfaces being bonded. Microballoons (WEST System 407) mixed into epoxy create a lightweight, easily sanded fairing compound — excellent for filling and fairing hull surfaces, but with much lower structural strength than silica-thickened epoxy. Never use microballoon filler in a structural joint — it's a fairing compound, not an adhesive.
Methacrylate adhesives (Plexus, Lord, Scigrip/Weld-On) are the other structural option, and they excel in specific applications where epoxy falls short. Methacrylates bond aggressively to plastics, composites, and metals — including surfaces that are difficult for epoxy, like polyethylene, polypropylene, and Delrin. They cure rapidly (10-45 minutes depending on formulation), develop high shear strength, and tolerate less-than-perfect surface preparation better than epoxy. In production boatbuilding, methacrylates have largely replaced epoxy for bonding deck-to-hull joints and structural panels because of their speed and tolerance for manufacturing variation.
Surface preparation for structural adhesives is non-negotiable. For epoxy bonds, both surfaces must be clean, dry, abraded to a fresh surface (80-grit sanding), and free of contamination. Peel ply fabric left on a cured epoxy surface during manufacture provides an ideal bonding surface when removed — the peel ply tears away and leaves a textured, contamination-free surface. For secondary bonds (bonding cured epoxy to cured epoxy), abrade with 80-grit and clean with acetone or denatured alcohol immediately before bonding. For methacrylates, follow the manufacturer's specific surface prep requirements — some formulations require only solvent wiping, while others need abrasion plus a primer.
When mixing WEST System epoxy, use calibrated pumps and verify the ratio by weight periodically. The pump system is convenient but not infallible — pumps can slip, develop air locks, or deliver inaccurate ratios if not maintained. An off-ratio epoxy mix will not cure properly and will produce a weak, potentially sticky laminate that must be completely removed and redone. A gram-accurate kitchen scale costs $15 and is cheap insurance.
Epoxy sensitization is cumulative and permanent. Repeated skin contact with uncured epoxy hardener can cause allergic sensitization — once sensitized, even trace exposure causes severe dermatitis. Wear nitrile gloves (not latex, which epoxy penetrates) for every operation involving uncured epoxy. If you develop a rash from epoxy exposure, take it seriously — continued exposure makes the reaction progressively worse and can eventually make it impossible for you to work with epoxy at all.
Anti-Corrosion Compounds and Lubricants
The marine environment attacks every metal surface on your boat, and the interface between dissimilar metals is where the worst damage occurs. Tefgel (from Tef-Gel PTFE anti-seize) is the gold standard for preventing galvanic corrosion at fastener interfaces. It's a PTFE-based (Teflon) paste that you apply to bolt threads, under screw heads, and between any metal-to-metal contact surfaces. The PTFE particles physically separate the dissimilar metals while providing lubrication that prevents galling. Every stainless steel bolt going into an aluminum fitting, every bronze screw going into a stainless plate — all of these joints should have Tefgel on the threads and under the head.
Lanocote (from Forespar) is a lanolin-based corrosion inhibitor that serves a different purpose. Where Tefgel prevents galvanic corrosion at fastener interfaces, Lanocote is a general-purpose moisture-displacing corrosion preventive for electrical connections, turnbuckle threads, shackle pins, swage fittings, and any metal surface that needs protection from salt air and moisture. Apply it liberally to turnbuckles after adjustment, pack it into wire rope end fittings, and coat the threads of every shackle pin on the boat. It is not a lubricant in the traditional sense — it's too sticky for that — but it is an outstanding corrosion barrier.
McLube (Sailkote and OneDrop) fills the lubrication role. McLube Sailkote is a dry-film PTFE lubricant that reduces friction on sail tracks, roller furling foils, traveler cars, blocks, and zippers. It dries to a slippery film that does not attract dirt or grime. McLube OneDrop is a penetrating lubricant for locks, hinges, and tight mechanisms. These products do not provide corrosion protection — they are friction reducers. Use them on moving parts and use Lanocote or Boeshield T-9 on static parts that need corrosion protection.
Lithium grease and marine-grade waterproof grease are needed for heavier-duty lubrication: winch gears, windlass gears, steering quadrant pivots, seacock handles, and any mechanical component with metal-on-metal bearing surfaces. Lewmar, Harken, and Andersen all specify their own grease for winch service, and using the wrong grease can attack the plastic and delrin components inside the winch. Standard automotive lithium grease is acceptable for most marine applications, but for winch internals specifically, use the manufacturer's recommended product — the cost difference is negligible and the risk of damaging expensive winch pawls and bearings with incompatible grease is real.
Boeshield T-9 deserves special mention as a versatile corrosion preventive and light lubricant developed by Boeing for aircraft use. It penetrates, displaces moisture, and dries to a waxy protective film. It's excellent for wire rigging, electrical connections, engine controls, and any surface that needs both corrosion protection and light lubrication. Many sailors use T-9 as their general-purpose spray and reserve specialized products (Tefgel, Lanocote, McLube) for specific applications.
Keep a small kit of anti-corrosion products in your on-board tool bag: a tube of Tefgel for any fastener work, a jar of Lanocote for rigging and shackles, a can of Boeshield T-9 for general protection, and a bottle of McLube Sailkote for running rigging and hardware. Having these products on the boat means you apply them in the moment rather than telling yourself you'll do it later — and later never comes.
Common Mistakes and Compatibility Issues
Silicone sealant below the waterline is the most common and most damaging sealant mistake. Standard silicone (RTV silicone) should never be used on a boat for any structural or below-waterline application. It has poor adhesion to most marine substrates, it cannot be painted over (nothing adheres to cured silicone), and it contaminates surfaces so thoroughly that no other sealant will bond to an area where silicone has been used until the substrate is ground back to fresh material. One tube of silicone used by an uninformed previous owner can create years of rebedding failures because the silicone residue prevents polyurethane and polysulfide sealants from bonding.
Mixing sealant types in the same joint is another common failure mode. Applying 4200 over old Life Calk, or putting Sikaflex over residual 5200, creates a layered joint where neither sealant bonds properly. Old sealant must be completely removed — mechanically with scrapers, then chemically with the appropriate solvent — before new sealant is applied. This is tedious, time-consuming work, and the temptation to skip it is strong. Resist that temptation. A joint with new sealant over old sealant residue will fail, typically within one season, and you'll do the whole job again.
Polyurethane sealants (4200, 5200, Sikaflex) are not compatible with polycarbonate (Lexan). The solvents and plasticizers in polyurethane sealants cause stress cracking in polycarbonate — fine cracks that propagate through the material, eventually causing it to craze and fail. If you are bedding polycarbonate portlights or windshield panels, use Sikaflex 295 UV (specifically formulated for direct glazing) or butyl tape. Many boat owners have destroyed perfectly good Lexan portlights by bedding them with standard 4200 or Sikaflex 291.
Using the wrong solvent for surface preparation is surprisingly common. Mineral spirits leave an oily residue. Lacquer thinner is too aggressive for gelcoat (it softens polyester). MEK works but is extremely toxic and often unnecessary. Denatured alcohol is the safest general-purpose cleaning solvent for gelcoat and fiberglass. Acetone is appropriate for metal surfaces and for final wipe-down of epoxy surfaces before bonding. The manufacturer's proprietary cleaners (Sika Aktivator, 3M Adhesive Cleaner) are formulated specifically for their sealant products and are worth the investment for critical joints.
Bed compound failure from overtightening fasteners is the most preventable sealant failure. When you crank down on a fitting bolt, you squeeze the sealant to a paper-thin film — or squeeze it out entirely, leaving metal-on-gelcoat contact with no sealant in between. The correct technique is to snug fasteners until you see a thin, uniform bead of squeeze-out around the entire perimeter of the fitting, then stop. Allow the sealant to cure at this compression level, and only then do a final gentle snugging of the fasteners. If there's no squeeze-out on one side, the sealant isn't making contact there, and that side will leak.
Before rebedding any fitting, test a small area with your chosen sealant and solvent on a hidden part of the substrate. This catches compatibility issues before you've committed to the full job. Apply the solvent, wait 10 minutes, and check for softening, discoloration, or crazing. Apply a small dab of sealant, let it cure overnight, and check adhesion by trying to peel it off. Five minutes of testing prevents hours of rework.
Summary
3M 5200 is a permanent structural adhesive for joints that will never be disassembled; 3M 4200 is the removable alternative for deck hardware; Sikaflex 291 falls between them — choosing the wrong one at installation creates expensive problems at removal.
Butyl tape is the best bedding material for deck hardware on cored decks because it seals effectively under compression, never cures, and allows easy removal for inspection and rebedding without substrate damage.
Structural epoxy (WEST System 105, TotalBoat) requires accurate mix ratios, proper temperature, and thorough surface preparation — colloidal silica creates structural filler while microballoons create fairing compound, and the two are never interchangeable.
Tefgel prevents galvanic corrosion at fastener interfaces, Lanocote protects static metal surfaces, McLube Sailkote reduces friction on moving parts, and Boeshield T-9 provides general-purpose corrosion protection with light lubrication.
Silicone sealant should never be used on a boat — it contaminates surfaces so thoroughly that no other sealant will bond until the substrate is ground back to fresh material.
Every sealant application requires complete removal of old sealant, proper surface cleaning with the correct solvent, primer where required (especially on teak and metals), and fastener tension that achieves squeeze-out without over-compression.
Key Terms
- Polyurethane Sealant
- A flexible, moisture-curing sealant that bonds strongly to most marine substrates. Available in permanent (5200), removable (4200), and intermediate (Sikaflex 291) adhesion grades. The dominant sealant type for fiberglass boat deck hardware and structural joints.
- Polysulfide Sealant
- A permanently flexible sealant (such as Life Calk) with moderate adhesion and excellent chemical resistance. Preferred for wood-to-wood joints and applications requiring long-term flexibility without the aggressive adhesion of polyurethane.
- Butyl Tape
- A non-curing, permanently tacky rubber sealant tape that relies on fastener compression rather than chemical adhesion to maintain a waterproof seal. Ideal for deck hardware bedding because it provides effective sealing with easy removal for maintenance.
- Colloidal Silica
- An ultra-fine powder (WEST System 406 or Cab-O-Sil) mixed into epoxy to create a high-strength, non-sagging structural adhesive. Used for bonding, filleting, and tabbing structural members. Not for fairing — use microballoons for that.
- Tefgel
- A PTFE-based anti-seize paste applied to fastener threads and under bolt heads to prevent galvanic corrosion between dissimilar metals. The PTFE particles physically separate the metals while providing lubrication to prevent galling during assembly.
- Squeeze-Out
- The thin bead of sealant that is pushed out around the perimeter of a fitting when fasteners are tightened to the correct compression. Uniform squeeze-out around the entire perimeter confirms complete sealant contact with no dry spots in the joint.