Marine Caulking Products Guide

3M 5200

Marine Caulking Products Guide will help you understand the difference and properties of the caulking options available and where to use the. Leaks can be just annoying, or cause structural damage and even lead to sinking. Despite this leaks are very common and can be found on new boats and old boats alike.

This page is dedicated to caulking products. Choosing the right sealant or caulk to bed fittings is confusing and a walk down the aisle at the store leaves you wondering what do I need. A sampling of caulking materials found at the chandlers, include fast and slow cure, permanent or removable, polyurethane adhesive/sealant, polysulfide bedding compound and silicone, as well as glazing adhesive, teak deck caulk etc., and too many choices.

Types of marine caulk and Manufacturers

3M 5200
3M 5200

Below is a list of the most common sealants you will find in a chandlery. Most of these sealants are high viscosity liquid sealants (except Butyl a tape) which are applied by tube, or caulk gun. They are applied like tooth paste. The liquid is able to easily spread onto a fitting and when pressured it flows into all voids.

Types of Caulk include;

  1. Silicone
  2. Hybrids
  3. Polyurethane
  4. Polysulphides
  5. Butyl Tape

Properties of Sealants

Curing; Curing may take days, depending on the material, air temperature humidity-most urethane sealants actually cure in the presence of moisture, which is why they’re excellent for emergencies underwater repairs. Many caulks are moisture cured; some are 1 part slow Curing times 2-14 days others are 2 part fast cure.

Stretch or Elongation; Elongation is a measure of Stretch. The higher the stretch the better the caulk bond will hold up. Caulk can have between 100 and 800% elongation and more at break.

Consistency; Liquid, paste or tape

Adhesive by the numbers; This chart gives you an idea of the difference between the types of major caulks. These numbers are a rough guide as products vary. Bear in mind 700psi could tear apart fiberglass when removing a fitting. Adhesion = peel strength, strength = tensile strength

  Polysulphide Polyurethane Silicone
Elasticity >400% 300-800% >350%
Tensile strength 140psi 300-700psi 220psi


1    Silicones

3m silicone3M Silicone

Silicones are inert synthetic polymer compounds. Primary used as sealers, with little bonding capabilities. Silicones can be used with almost any material and is an excellent insulator between metals. There are 2 types of silicone; One is based on acetic acid (vinegar smell), while the other is ammonia-based.

Manufacturers Examples; 3M marine grade silicone sealant, Boatlife silicone rubber


  • Silicone is a gasket material
  • Use above waterline only
  • A weak adhesive
  • High elasticity
  • UV stable
  • Safe for plastic fittings
  • Silicone sealers cannot be painted.
  • Forms chemical-resistant gasket.
  • Good insulator

Disadvantages; Any trace of silicone will cause problems with paint. Silicone runs the risk of contaminating surfaces to be painted or varnished. Weak adhesive Only for above waterline.


2 Silicone/Polyurethane Hybrid

boatlife lifeseal
boatlife lifeseal

boatlife lifeseal


  • Better adhesion than silicone
  • Excellent for port lights.
  • Silicone sealers cannot be painted.
  • Less aggressive than straight polyurethanes

Applications Ports, glass, plastic,

Disadvantages Low adhesion

Examples BoatLife Life Seal,



3   Polyurethanes

3M 5200
3M 5200

PU Polyurethane is the sealant when you are looking for a permanent bond. Excellent for hull to deck and hull to keel joints.

Examples; 3M; 5200 & 4200, Sikaflex 292, Sikaflex 291,


  • Not for fittings that may need to be remove
  • Below waterline use
  • Can be painted
  • Polyurethanes not UV stable unless stated
  • Not for polycarbonate, acrylic, ABS, and PVC Although PU attacks plastic,
  • Marelon thru hulls are OK

Disadvantages; Stay away from 5200 unless you are looking for a permanent bond Solvents in Polyurethane attacks plastics, like polycarbonate, acrylic, ABS, and PVC.


4   Polysulfides PS

Boatlife lifecalk

Boatlife lifecalk

A synthetic rubber with excellent adhesive properties. A good caulking compound that has a combination of adhesion and is flexible. The best choice for most deck fittings except plastic

Examples; 3M Marine Sealant 101 Life calk by Boatlife


  • Both good adhesives and good sealants.
  • Bonds well to teak and primed oily woods
  • Polysulfides have good resistance to UV
  • Polysulfides also possess excellent resistance to petroleum products
  • Can be painted.

Disadvantages Solvents in Polysulfide and Polyurethane will soften some plastic, Do not use with Plexiglas or Lexan ports.

5   Butyl Tape

Butyl Tape
Butyl Tape

Butyl Tape

Butyl tape is very easy to use Temperature; sticky in warm, hard in cold Butyl is a flexible gasket Butyl tape is convenient to use Butyl is NOT an adhesive Do not use where bonding is required, No curing, waterproof immediately

Disadvantages Disadvantages include the tape is not impervious to gasoline, or diesel,

Sticky in high temps, does not flow in cold temps. Best use is at 70 degrees


Other Caulking compounds

Polyethers PE; 3M 4000 is an example and are new and have very good UV resistance. Their lack of solvents minimizes shrinkage.

Plexus; used by Tillotsen Pearson and J Boats. Plexus is so good an adhesive J Boats use it for Hull to Deck joints and do not use any mechanical fasteners.

Home improvment Caulk; These tend to be cheaper, what a surprise, however many of them are acrylic based and do not have the adhesive qualities. they also shrink and break down in the marine environment. You get what you pay for at least sometimes.


Choosing the right caulk for your application will be simplified by answering these questions; Above or below waterline, Do you need bonding or just a gasket, Will you remove fitting later, are fittings plastic.

The table below breaks down typical maintenance projects into categories above and below the waterline, with the recommended products from major manufacturers.

  Application 3M BoatLife Sikaflex Dow
Above waterline Metal Deck fittings 4200 or 4000 UV Life-calk 291 or 292
Metal Port hatch 4200 or 4000 UV Life-calk 291 or 292
Plastic ABS or Lexan Ports & Hatches LifeSeal 295 UV Dow 295
Electrical fittings 3m silicone Silicone rubber & Lifeseal
Windows glazing panels LifeSeal 295 UV or 296 Dow 795
Below waterline Hull keel joint or Hull deck joint 5200 or 4200 Life-calk 292
Thru hull Hull keel joint or Hull deck joint 5200 or 4200 Life-calk 292

Surface Preparation

Without proper surface prep any caulk will not live up to its potential. Surface preparation; Clean well to remove dust, wax oil and all contaminants. In some cases you could sand with 180 or 220 paper.

Follow the manufacturer instructions for prepping the surface before applying sealant; some manufacturers have primers, notably Sikaflex.

Applying Caulk

Application; Most Caulk comes with thier own dispenser. Small tubes have nozzles that screw on the top. You can cut the nozzle to apply the caulk. If you cut further down the nozzle you can apply the Caulk quicker, or if you just want a small amount of Caulk cut closer to the top.

Viscosity; Viscosity of Caulk tends to be like treacle. It is fairly thick so it does not pour out of a joint.

Lower Viscosity Caulk; If you find the sealants are too sticky and do not flow enough the Liquid version of LifeSeal is self-leveling. If you need to get sealant in a hard to get to spot, this sealant will flow. BoatLife Liquid Life Sea