Article by Capt. Steve Katz
Sticky Business
When it comes to boat repair, maintenance or even boat building, there are many methods used to join and seal materials together.
Traditionally, pleasure boats had mechanically fastened joints and connections held together with self-tapping screws or through bolted with nuts and bolts. Modern boats are built with a variety of materials and require a variety of different fastening methods, often taking advantage of modern adhesives and sealants in addition to mechanical fasteners. Some production boats even use special adhesives such as methacrylate adhesive in place of traditional mechanical fasteners for structural joints.
Choosing the correct adhesive or sealant should be done carefully, since products are often intended for a specific purpose and used in the wrong situation or incompatible materials will yield poor results or even a joint failure.
Adhesives vs Sealants
Sealants are used to seal joints, gaps, and cracks between two different surfaces. These surfaces are mechanically fastened. While the sealant needs to have some limited adhesive properties, enough for the sealant to stick to the surface of the materials but not designed to bond the materials. Some sealants are approved for use below the waterline, some sealants include UV stabilizers that reduce failure when exposed to sunlight, some sealants are for indoors only, some sealants can be painted over, some sealants can cover a wide gap, each type of sealant has its own properties, be sure to use the correct product for your needs.
Adhesives are designed to bond two materials together. Most adhesives have specific properties where they perform best and often function as a sealant too. There are many types of adhesives designed for the materials that are intended to be bonded together. Using the wrong adhesive for the materials can result in inferior bonding or even damage to the material.
3M 5200
Walking through a boatyard or marina you cannot go far without seeing a tube of 3M 5200 or a blob of the white ubiquitous paste stuck somewhere it should not be! This is the most popular but often misused adhesive/sealant product in boating, let us see why:
What makes 5200 such a popular product? It can seal and adhere to almost anything! This paste is sold in various size tubes and the manufacturer describes it as a one-component, high-strength, moisture-curing, gap-filling polyurethane for permanent bonding of wood, gelcoat, and fiberglass. It cures to form a firm, rubbery, waterproof seal on joints and boat hardware, above and below the waterline. 3M indicates that this product can be used to bond: Fiberglass deck to hull, through hull fittings, wood to fiberglass, portholes and deck fittings, motors on fiberglass transoms and hull seams above and below water line. Based on 3M’s description, this product seems to be a one-size-fits-all making it popular BUT it may be too good for everyday use on a boat!
Tensile strength is “the resistance of a material to breaking under tension” or a measure of the bonding strength of a material. Tensile strength is one of the significant differentiation properties between 3M 5200 and other related products by 3M and competitors. 3M 5200 has one of the highest tensile strengths of any marine adhesives at 600 psi and 3M indicates that it should be used for permanent bonding, meaning that the two materials are not intended to be separated (ever again!). Many boaters have learned this the hard way when they try to dissemble bonded items with much difficulty and often damage the substrates.
5200 Alternatives
Comparing 5200 to another popular 3M adhesive, 4200FC, a polyurethane chemical base product which has a 180 PSI tensile strength, designed to work above and below the waterline as a sealant/adhesive and is formulated so that the two items can be disassembled (for replacement, service, repair etc.). This is often a better choice than 5200 for most projects.
3M also offers a similar product, a Marine Adhesive/Sealant 3M 4000 UV is a one-part Silane-modified polymers (SMP) adhesive sealant that cures to form a firm, rubbery waterproof seal. 4000 has a tinsel strength over 400psi and is recommended for above and below the waterline. Its superior UV resistance properties make this an ideal cosmetic adhesive sealant.
Silicone
Another popular product used on boats to seal and adhere is marine silicone, the 3M marine grade product has a 220 psi tensile strength, slight higher than 4200 but is designed for above the waterline and is not paintable (most paint does not stick well to silicone) Marine silicone has one of the largest elongation rates of all marine sealants and adhesives, able to stretch without breaking, up to 350% of the original size, great for joints move and flex.
Bedding
One overlooked sealing product is a “bedding compound.” A bedding compound is a heavy bodied compound for bedding-in deck hardware, molding, and other items where a waterproofing compound is needed that is not an adhesive. Items need to be mechanically fastened before being bedded. The popular Pettit Dolfinite Bedding Compound can be used on canvas, glass, metal, fiberglass, and wood or wherever two pieces are fitted together, and a waterproof, dustproof seal is desired. Bedding compounds remain soft and flexible, though the exterior surface of a bedding compound skins over and can be painted to match the adjoining areas.
The variety of adhesives and sealants in a marine supply store seems endless, choosing and using the correct product for the job is critical, most manufacturers offer detailed technical documentation that can assist in selecting the right sealant or adhesive for the job – read up on the products before you begin your project.
NOTE: Airmar- the popular manufacturer of depth sounder transducers recommends Boatlife®'s Lifeseal® or 3M 4200 as a sealant/adhesive during the installation of many of their products and not 5200.
NOTE: Read the manufacturer's instructions for surface preparation, application, and curing for any sealant & adhesive. For example, the 3M 5200 instructions indicate that “Alcohol should not be used in preparation for bonding as it will stop the curing process, causing the adhesive to fail.”
NOTE: if you are working with HDPE plastic, such as Starboard, there are not many products that will adhere or seal to Starboard, including 3M 5200, as indicated by the manufacture of Starboard, King plastics. King recommends mechanical fastening or welding, though if needed, the following adhesives including Lord 7542-AB, Scotch-Weld DP-8005, or Chem-Set™ 6105 Polyolefin Bonder could be used with Starboard, carefully following King's preparation procedure. §
