Kayak Building Bulletin Board Old Message Archive


polyester resin history

Posted by: Paul G. Jacobson on January 5, 2000

> I want to build a low cost stitch and glue and have been given polyester
> resin and cloth free. My question is how do I use these items instead of
> epoxy resin and how do i scarf joints without using epoxy. If i glass the
> wood do i need okoume or will inexpensive birch lauan do the trick but
> just add weight?

Get a waterproof (not water resistant) glue for your scarf joints. This can be epoxy resin if you wish. Otherwise, there are several varieties of glues on the market which you can use. It seems strange, but if you can use it straight from a single bottle it probably isn't going to be waterproof, and if you have to mix it before using it, it probably will be waterproof. You won't need much glue for the scarf joints; 8 ounces of glue should be enough.

Go ahead and cover the boat with your free glass and polyester resin. 15 or 20 years from now you may want to refinish it -- or it might still look like new! Worry about that in the far future.

Until recently ( past 10 to 15 years) polyester resin was the only resin commonly available. In fact it may still be more widely available than epoxy, mainly because it is sold in so many automotive shops. A lot of boats were built with this material.

The person who claims to have invented stitch and glue techniques developed them using polyester resin. Clark Craft's parts catalog (at least my 5 year old copy) suggests using them for stitch and glue, and they sell a thickened polyester for use in making the fillets. If you can, find books by H.H. 'Dynamite' Payson.

Old plans for cedar strip canoes called for using WHITE cedar, as this wood was more compatible with polyester resin than RED cedar, which was an oilier wood. Well, nowadays, RED cedar is sold widely for use in making decks and porches, while WHITE cedar is a special order item. People who choose to build with the commonly available wood have gone to epoxy because it works with RED cedar as well as many other woods. David Hazen's book on cedar strip canoe building covers some of this, as well as a technique for sealing ther RED cedar so it can be used with polyester.

You are not building a strip canoe (or kayak), so you don't need to worry about the old issue of cedar species. Pine, fir, and spruce -- the main woods used in common construction plywoods -- adhere fine to polyester resin. Lauan should work just fine, too.

There was a response that mentioned forgetting stitch and glue and building with chines and ring nails. Let me point out that these construction techniques are essentially identical. stitch and glue's technique is basically the installation of a CUSTOM MOLDED, glass-reinforced-plastic chine in the location where a custom fitted wood chine would conventionally be located. By using the wires to temporarily hold the pieces of the boat together, you create the boat shape, but without the usual chines -- which are pieces of wood used for joining these hull panels. After the boat shape is made, though, the fillet of reinforced plastic is applied, and it hardens to a greater strength than wood of a similar size. by covering the fillet with a piece of 2 inch (or wider) fiberglass tape you not only increase the strength of this molded plastic chine ( and that fillet really IS a chine when you are finished with it) but making it thicker, but you are also increasing the surface area of the bond between the boat's wood and the plastic chine. Think of wall paper: A very weak glue made from a flour paste works well for decades because it is over such a large surface area. Or, think of painting around windows. Paint is an awful glue, but if you cover enough area around your windows with it, that window will not open easily when the paint dries. The same enginering principle seems to apply here. Polyester is not a great glue, but you dont need much adhesive force to hold on a perfectly molded chine, and the polyester is by far strong enough to meet that challenge.

So go with it. Get the boat built, use it, enjoy it, and be happy that you are using a material that is not as sensitive to UV light as epoxy is. You DO need to protect epoxy from UV. Polyester does not need such added treatment.

When using the polyester remember that there are two types. One has a wax dissolved in it, and the other does not. Polyester will not set up and harden properly in the presence of air, so when you put on the glass cloth you just have to wait until the resin is reasonably solid, even if it is still tacky, before you put on the second and third coats to fill the weave.

If your resin has wax dissolved in it and you keep working with thins while the surface is still tacky you don't need to do any sanding between coats. On standing, the dissolved wax covers the surface of the resin to exclude air and allow the material to harden. If your materials do not have dissolved wax then you need to put on a special layer of material to exclude air from the final coat of resin. You can buy special resin for the final layer, you could mix your own parrafin in (get it at the grocery store -- food canning section) or you can spray on a coat of a mold release compound like polyvinyl acetate, which can wash off with water, or you can cover it with plastic (like saran wrap) or put it in a large plastic bag and suck all the air out with a vaccuum pump -- which is commonly done in industry. You don't have this minor hassle with epoxy.

good luck with your building. Hope this information helps

Paul Jacobson


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