Four types of boat glue and one powerful sealant
Modern glues have proved to be surprisingly efficient and durable, even on boats, where salt, moisture, heat, grime, and the ultraviolet (UV) rays of the sun do their best to degrade them. Many boat parts that formerly required fixing with nails, screws, rivets, or nuts and bolts are now fastened in place with glue only. Indeed, the major hull components of many small plywood boats are now held together solely by fiberglass tape glued in place.Nearly all glues used in boat work fall into one of the following four categories:Rubber glues, the basis of contact adhesives, may be natural or synthetic.Melamine-urea types of glue are water-resistant and very forgiving, which makes them suitable for amateur use. Examples include Weldwood plastic resin, Casco urea-formaldehyde, and Aerolite resin glues.Epoxies, probably the most popular glues used on boats, also are forgiving in many ways because their curing times can be controlled, and they can be stiffened and strengthened with fillers to avoid dribbling and slumping. Epoxy resin fills large gaps and is an excellent sealer for wood. It is almost exclusively used to laminate new fiberglass to old fiber-glass hulls.In fact, epoxy sounds like the ultimate glue, and it might have been except for two flaws: in its normal state, it’s not waterproof or heatproof; and it can be adversely affected by salt water and sunlight, both of which are plentiful where boats abound. For exterior use, therefore, it should be protected from salt and sunshine by paint, preferably white or a light color. Varnish that contains UV-ray filters offers some protection, but not as much as paint.Resorcinols have the very qualities that epoxies lack: they are not affected to the same extent by water or sun. Typically a purple-brownish color, they are the glues used in exterior- and marine-grade plywoods. The original resorcinols were not gap-filling and required a good fit between the pieces of wood to be joined. However, newer varieties have the ability to fill gaps and require less clamping, making them the best wood-to-wood glue available for marine use.Resorcinol comes in two basic forms. One needs temperatures of 70°F (21°C) or higher to effect a cure; the other, an imported version made by Ciba-Geigy and known as Aerodux or Cascophen, will cure in temperatures as low as 50°F (10°C). The U.S. product is known as Weldwood and is available at marine hardware stores.Polyurethane sealant needs to be mentioned in connection with glues. It’s a bedding compound that also has great adhesive powers—so much so that anything you bed down in polyurethane is never likely to come apart. It’s used for permanent sealing bonds, such as hull-to-deck joints, or where a ballast keel joins the hull. The most popular brand is 3M 5200.See also Sealants.