: I was reading on another board that when you epoxy-glass a boat, you deprive
: the wood of the ability to breath naturally. This was their thinking
: against building stich and glue boats. It is my belief that strip built
: boats also employ epoxy coatings. They contend that epoxy always leaks a
: bit, trapping some moisture and thus causing rot. Is any of this true?
: Does epoxy-glass leak a bit and does it cause some kind of rot? Is there
: any difference in this regard between a stripper and stich and glue in the
: way it employs epoxy and glass? I am trying to decide between these two
: systems and I want my end product to be long lasting.

This post, and at least one response, seem to have a few misconceptions, which I will try to put to rest.

First, there seems to be some confusion about the resins used in fiberglassing boats. Clark Craft has an interesting discussion about the differences between polyester resin and epoxy resin in either their catalog or their free price list. I don't have either at my side at the moment so I hesitate to suggest anyone spend the $5 to buy their catalog jsut for this discussion. I believe you can find similar information in materials available from Fibreglast and other resin and glass-fiber retailers as well.

Whwn you see a "fiberglass" boat, motorcycle helmet, archery bow or chair what you are probably looking at is a product made of a p;astic that has been reinforced with pieces of glass that are so thin they appear to be like silk or cotton fibers. The generic name for these types of materials is Fiberglass Reinforced Plastic, or FRP. While the two go together well, they are not by any means an exclusive pairing. You have undoubtedly seen plastics without reinforcement, and fiberglass woven into fabric used as draperies. You can use other materials to reinforce plastics, and you can use fiberglass to reinforce rubber or concrete, too.

Glass fibers can be used with almost ANY kind of plastic, but the materials WE frequently work with are plastics that we can get in a liquid form, and which will change into a solid form through a chemical reaction. There are three classes of materials on the market that are commonly available and widely used. The oldest, or at least the one that has been used the longest, is polyester resin. The others are vinylester resin and epoxy resin.

Generally, products that we call "fiberglass" are made from polyester resin. Industrial use of this stuff is extensive, and in large quantities, or on sale, it is about as expensive as a good quality paint -- I spent $14 for a gallon of the stuff last year. Epoxy, on the other hand typically sells for $35 to $65 a gallon. The price of vinylester falls between these.

The chemical reaction that converts these liquids into solids is called polymerization. Basically, something starts a reaction in the liquid chemicals which causes them to harden. As the resins harden, the molecules connect with each other and 'grow' into interlinkied chains of molecules which are very stable and strong. Polymerization is all around us: A similar reaction can be observed by cooking an egg in a frying pan. As the proteins in the eggwhite and the yolk get hot they become solid (and arguably, more appetizing). When you paint a house with an oil bsed paint, the oil oxidizes slowly, congealing and solidifying as the paint 'dries'.

Epoxy resins typically mix a hardening agent with a resin. The combination of these two components triggers the polymerization reaction, which is accompanied by the production of some heat. In turn, the heating helps to speed the reaction. With a good quality of epoxy the resin and the hardener have almost no added volatile solvents and the entire batch of material sets up to form a solid plastic material.

Polyester resin simply needs an oxidizing agent to get started. Strangely, it generally will not polymerize properly in the presence of air. While the interior of a plastic part will harden, the surface will stay sticky. This is a wonderful attribute if you are going to make a thick part, as you can build up layer after layer of material, and the sticky old surface will bond chemically to the freshly applied new material. For tha lst coat you simply seal off the air by applying a coat of wax, styrene plastic, or something similar. Polyester is thick, like honey, and occasionally solvents are added to lower the viscosity and get it to flow more easily. These solvents have to evaporate as the material hardens. In addition, when the hardener reacts with the polyester resin a small amount of gas is produced. This gas also evaporates readily as the resin hardens. Unfortunately, as the solvents and gas leave the plastic they leave behind a microscopic trail - essentially a threadlike hole - which is so tiny that water molecules can not pass through.

At this point I want you to look up all you can find about Gore-tex. They can do a much better job of explaining the physics of why water vapor can pass through the tiny holes in Gore-tex, but liquid water drops can not. It is something about surface tension and molecular size. It is the same with polyester resin, only the 'holes' left by the escaping gases and evaporating solvents are much smaller than those in Gore-tex. Water can not pass through polyester plastics, but over a period of time, water vapor can. How much? Well, enough so that if there is wood on the inside of a coating of polyester resin, AND if that wood has spores from molds and fungi that cause rot, then over a period of time rot MAY develop under the layer of resin.

Actually, almost all paints have the same tiny holes from the evaporation of their solvents as they dry, and so do varnishes and lacquers. Putting varnish on an epoxy surface has no effect on the transmission of water vapor, as the varnish 'breathes'. The epoxy, though, does not develop these tiny holes and is both waterproof and vaporproof. Wood sealed under epoxy does not absorb water vapor, and thus cannot rot as long as the seal is good. Of course a decent scratch that goes through the coating is going to allow the exposed wood to wick up water like an inefficient sponge, so don't assume this is totally failure proof.

Epoxy resins are more sensitive to UV (Ultraviolet) light than Polyester resins. Over time epoxy plastics will deteriorate if left under strong sunlight. To protect against that we usually paint over epoxy resins with opaque paints (which block ALL light), or with varnishes that contain materials which reflect or absorb UV light, and prevent it from passing through. the varnishes and paints are less durable than the epoxy and need to be replaced at regular intervals. Varnish lasts a year or two. Paint may last 5 or more years.

Epoxy resin, being a bit "thinner", or less viscous, soaks into the fibers of the wood. If it is applied under ideal conditions it may soak in as deeply as 1/16th of an inch, or more. That would be 1 to 1.5 millimeters. Polyester resin however, is thicker. It sits on the surface of the wood and hardly soaks in at all.Since the polyester is thicker, it sags, drips and runs less on vertical surfaces, which makes it convenient for use on curved boat surfaces.

Wood which has received two coats of epoxy resin - the first soaking in to seal the grain and pores in the wood, and the second to provide a seamless coating -- is as resistant to water absorbtion and rot as you are going to find. Polyester does a better job than most other materials, including paint, tar, creosote, etc. and has a long service life, but rot can develop under it eventually. How long will it take for this to happen? With average use, where the boat removed from the water after use each day, probably never. If you store your boat in the water, there is a chance it might develop some rot eventually.

OK, so we are thinking or these plastics as if they were paints. Where does the actual fiberglass fabric come to play? It serves several functions. By wrapping the glass cloth around the outside of the boat you create a strong skin with incredible tensile strength that serves as an exterior clamp, holding all the parts together. Likewise, a layer of cloth on the interior strengthens the hull against punctures and those forces genereated when hitting rocks and obstructions.

Generally, the thickness of the fabric determines how strong this skin is. thicker fabric gets that way by having more glass threads. as the fabric gets more fibers and its thickness increases, so does its weight. It is sold by the weight. A square piece of fabric that is one yard on each side is weighed. Typically we materials that are 4 ounces or 6 ounces per square yard. In the metric system the cloth is also identified by its weight, but they use a different base size (probably a square meter).

The thickness of the fabric also serves as an embedded depth gauge for the plastic resin coating. If the fabric is 1/100th of an inch thick, then the resin coating will seep through the fabric, and pool up on underlying wood. Additional coats of resin eventually cover the entire thickness of the fabric, concealing it's woven texture, and ensuring that the coating is at least 1/100th of an inch (just an example here) over the entire boat.

Capillary action and the wells formed by the weave in the fabric help to keep the resin from dripping and sagging.

The thickness of the plastic and the strength of the glass cloth help to prevent punctures. You trade weight for resistance here. Obviously a 6 inch thick layer of these tough plastics will be must more resistant than a 1/100th inch thick layer, but a boat with a 6 inch thick hull will be darn heavy! Some people put additional layers of fabric over high stress areas. When coated with plastic resins these thicker areas offer greater 'ding' resistance.

polyester and epoxy resins both work as glues, but epoxy is a superior glue. In stitch and glue construction it is the bonding force ( gluing strength) of the resin which holds the boat together at the seams. Some stitch and glue boats are not covered with fiberglass, or epoxy resin. These are typically older designs. Since the wood is not covered with fiberglass cloth, the boat is just painted.

Older wooden boats were constructed with relatively loose seams, which were packe, or caulked, with a fibrous filler. The paint which covered these allowed moisture to enter, swelling the wood fibers and that caulking to create a watertight seam -- as long as the boat stayed wet. When rot would develop after a period of years, owners of these older boats would try to solve the problem by sheathing their boats with plastic. Years ago they used polyester resins as it was the only product on the market -- and it was superior to paint. Well, with the old, saturated wood under the polyester coating rot could continue to spread, and these coatings got a bad rap. Using epoxy would probably not help much in this case, either, as the wood would have been covered with sealers and paints over the years, so the epoxy could not soak in to seal the wood effectively.

With woodstrip construction each strip is held to its neighbors by a thin bead of glue on the thinnest edge of the strip. This suffices for construction, but these edge-glued strips are not strong enough to endure the rigors of paddling. So, we wrap them in a casing of glass cloth and resin. The resin acts as a surface bonding glue. It does not have to be exceptionally strong as the entire boat is covered. That is a lot of square inches of coverage, and a lot of strength in holding things together.

The predecessor to stitch and glue construction used thin strips of wood, called chines, at each seam. Using screws, these chines were drawen tight to the panels that made up the hull, sides, deck, etc. A layer of fabric soaked in a waterproof glue served as a gasket between the chines and the panels.

With stitch and glue, thin wires hold the panels in place while a plastic 'chine' is created in the inside of the boat. This is done by molding a thick paste of plastic resin along the entire length of the seam. Frequently a layer of fiberglass fabric which has been woven or cut int the shape of a long ribbon is applied over this plastic chine and bonds it firmly into place. A layer of fiberglass cloth on the exterior seals the seam edges on that side. Polyester was strong enough for this job in earlier plans. Now people use epoxy as it is a stronger glue. In either case the bond was usually stronger than the wood that surrounded it.

It is late and I may be rambling a bit (please pardon me for that), but I hope this gives you some information and answers your questions.

Build a design you like. If it calls for strip building, then go that route. If it can be made from plywood parts, then use plywood and stitch together those panels. If you need to save money now, use polyester resin. Twenty years from now you can decide whether you need to refinish it. If you use epoxy, then be sure to protect it from UV. You'll need to refinish this each year or two by putting on more layers of UV resistant varnish.

It is 6 of one, and a half dozen of the other.

Paul G. Jacobson

Paul G. Jacobson