Marine sealants and silicones: which one to use where on your boat

Our top 3 picks

Stock checked on October 6, 2026

Top rated

3M Marine Adhesive Sealant Fast Cure 5200 (05220) White, 3 fl… 9.0 ★★★★★ BarcoEquipado score: 9.0 out of 10 See price

Best seller

3M Marine Adhesive Sealant 5200, Black, 10 fl oz Cartridge with… 8.7 ★★★★★ BarcoEquipado score: 8.7 out of 10 See price

Best value

Meguiar's M2501 Marine/RV Hybrid Ceramic All Surface Sealant… 8.1 ★★★★★ BarcoEquipado score: 8.1 out of 10 See price

marine sealants and silicones comparison, in stock

See more options on Amazon →

BarcoEquipado score: combines the average rating and the number of buyer reviews on Amazon. No brand pays to be featured.

Types of marine sealant and what each is for

Poorly bedded hardware shows no sign of trouble in the first year. Water creeps in drop by drop along the base, soaks the deck core and corrodes the fasteners. Picking the right product is half the job.

Polyurethane (PU)

The classic marine sealant: it bonds well to gelcoat, wood and metal, stays flexible and handles salt water. There are two families, semi-permanent (seals and bonds, but the part can still be removed later) and permanent (close to a structural adhesive). The weak spot is sunlight, since some types yellow with long exposure. Standard polyurethane is also not compatible with acrylic or polycarbonate.

Hybrid polymer (MS / STP)

Excellent UV resistance, no isocyanates, bonds without primer to many materials and can be painted. A great choice for joints in full sun. Structural strength is usually a bit lower than a permanent polyurethane.

Silicone

It handles heat and UV well, but adhesion is poor, it cannot be painted or sanded, and it leaves the surface contaminated so nothing else sticks afterward. Use it only for low-stress interior joints, and only neutral-cure types; acetic (vinegar-smelling) silicones can attack metals.

Teak deck caulk and butyl tape

Deck caulk is made for teak seams: flexible, sun-resistant and sandable. Butyl tape never cures, stays pliable and lets you remove hardware easily. Many riggers prefer it for bedding stanchions and cleats.

Which sealant to use for each job

Application	Recommended type	Avoid
Stanchions, cleats, hardware bases	Semi-permanent polyurethane, MS hybrid or butyl tape	Silicone; permanent adhesive
Through-hulls and transducers below the waterline	Marine polyurethane rated for immersion	Silicone
Hull-to-deck joint	Permanent polyurethane	Silicone
Acrylic or polycarbonate ports and windows	Glazing-specific sealant, with primer if the maker requires it	Standard polyurethane (it crazes the plastic)
Teak deck seams	Sandable deck caulk	Silicone
Rub rails and trim in full sun	MS hybrid or UV-resistant polyurethane	Acetic silicone
Galley sink, shower, interior joints	Neutral silicone or MS	Acetic silicone near metal

How to bed deck hardware so it does not leak

1. Remove and clean. Strip all the old sealant and degrease with a solvent that is safe for gelcoat. Nothing sticks to silicone residue.
2. Seal the core. On cored decks, dig out a little of the core around each hole, fill it with epoxy and re-drill. Then water cannot reach the core even if the sealant fails.
3. Countersink the hole edge. A small chamfer in the gelcoat gives you a ring of sealant around the screw and prevents cracking.
4. Mask with painter's tape around the base so the squeeze-out stays clean.
5. Apply sealant to the base of the fitting and under each screw head.
6. Snug it down, do not crush it. Tighten just enough for a little sealant to ooze out around the edge, then do the final torque once it has partly cured. Cranking it down fresh squeezes out the sealant and leaves a joint with no thickness.
7. Trim the excess and pull the tape before the sealant skins over.

Fasteners matter as much as sealant: a screw that is lower grade than the fitting rusts and breaks the seal. See our marine stainless fastener guide.

Common sealant mistakes

• Silicone on deck hardware: it leaks early and contaminates the surface for good.
• Permanent adhesive on removable parts: you rip out gelcoat when you remove them.
• Standard polyurethane on acrylic: it causes tiny cracks that spread over the years. Also drill the hole slightly oversize so the plastic can expand.
• Sealing over a wet, greasy or old-residue surface: adhesion drops sharply.
• Working in the cold: below roughly 40-50 °F most products cure very slowly; check the data sheet.
• Leaving the cartridge open: the nozzle cures within hours. Cap it with tape or a screw.

Tools and the yearly check

Besides the sealant you need a good cartridge gun (cheap ones flex on thick products), painter's tape, a utility knife, plastic scrapers, nitrile gloves and a spatula to tool the bead.

Once a year, ideally while winterizing the boat, inspect the bases of stanchions, cleats, blocks, ports, through-hulls and the edges of hatches. Look for cracked, lifted or blackened sealant, rust streaks running out from under a base (trapped water) and soft spots around fittings. If the gelcoat has cracked around a fitting, repair it before resealing: how to repair gelcoat step by step.