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> Hi,
> I am playing around with an inexpensive polyester epoxy (Bondo).
Polyester and Epoxy are two different types of plastic resins which are commonly used for our boatbuilding. There are many more resins on the market, but these two, and a third -- vinylester resin -- seem to be the best choices for our purposes. Since they have different properties and costs, builders and designers will select what they need for a given design.
A lot of times you can use either material. This is particularly true with older designs that were geared for use with polyester resin.
When epoxy hit the scene it was recognized as having some superior characteristics. If a boat REQUIRES those characteristics, then going with polyester could be a big mistake. Or, you just might get a boat that is perfectly fine on lakes, but not suitable for shooting rapids -- and you won't know this until you are already in the middle of some rapids and discover you have started swimming, while wearing just your cockit coaming and sprayskirt.
> I joined
> two plywood pannels using this resin mixed with cabosil (I guess it is a
> powder made of fibers). I joined them using the S&G technique. I also
> saturated few pieces of plywood. The result is every thing looked very
> strong. I know many people don't like using polyester because it
> delaminates, absorbs moisture. and it is not as strong as epoxy.
I think you have nailed the common complaints about polyester. It does not really absorb a lot of moisture, but it is porous on a scale so microscopic that water vapor can slowly pass through, but liquid water molecules can not.
It is the water that gets between the resin and the wood that leads to eventual delamination. This might take 20 years or more, or it might never happen. Or, with either epoxy or polyester, you could get a deep gouge on the first launching of your boat, allowing water to enter the wood fibers and cause a delaminated area. Regardless of the cause, delamination occurs in patches -- which can be fixed. This is just part of the upkeep on a wood boat.
Chewing gum, bailing wire and duct tape have been used to hold airplanes together, but you will never see them as recommended repair parts. Polyester resin is not usually recommended as a glue for wood. There are better adhesives for about the same price -- but the resin DOES stick to a lot of things, including dry wood. The manner in which it is used in stitch and glue construction, or fiberglassing a hull, is time tested and well proven.
Epoxy, on the other hand, has a long history as a glue, and I, personally would use that before I would consider using polyester resin as a glue.
Most of the `fiberglass' materials you see, from Corvette body panels to chairs at the bowling alley, are make of what is generally called FRP, or fiberglass reinforced plastic. In many cases the plastic that is being reinforced is created from polyester resin.
When you are talking about reinforcing materials there are products like carbon fibers, Kevlar fibers, glass fibers, and fibers from dacron, nylon, etc. Even polyester plastic, in a fiber such as in used in clothing (got shirts of 50% poly/50% cotton, or 100% poly?) can be used as a reinforcement. Some of these bond better with epoxy and some bond fine with polyester resin. Suppliers of these materials can tell you what works. When they work with polyester, then that is usually the resin most industries choose. The big reason seems to be cost.
> Here are my questions: 1) how strong is strong enough?. Is the polyester
> strong enough for kayaks.
Second part first: Yes, polyester is strong enough for kayaks. (and canoes) There is a long history of use of this material and a great amount is known about its characteristics -- both good and bad.
As for how strong is `enough`. There are many passionate notes in the archives of this list. Take a month and read them. My personal take on the whole matter is that no one really knows any definite numbers. I do have some personal benchmarks, based on historical and anecdotal evidence.
Here is how I personally look at the matter: Birchbark and skin canoes and kayaks were used for centuries. They work. Any canoe hull made from a material that exceeds the characteristics of a piece of birchbark should be fine. Will it break if you hit a rock? Maybe, but if it does then yoou avoid hitting rocks in the future. Meanwhile, you repair your boat and keep using it.
Rubberized canvas and other rubberized fabrics have been used in kayak hulls for over 90 years ( see the Klepper history at their webpage www.klepper.com). Will these puncture if you hit a rock? Maybe, but if they do you patch the hole and continue on your trip. And if they don't it is because they deflect , and let you bounce off the obstacle. So, any kayak skin that is stronger than canvas should be strong enough. Actually, I have a couple of inflatable boats made from a vinyl that is far weaker than the canvas I used on my kayak, and they have a decent service life.
> 2) Are there any data that show that polyester is not suitable for Kayaks.
> I read a few books on boat building and all that I found on polyester are
> general statements whith no backup data. Has anyone tried it?
Last question: Yes, many have tried it. And liked it! And some of them have gone on to epoxy and liked that more.
First question: No. The opposite is true. There is a lot of data showing that polyester is quite suitable for kayaks. The inventor of the stitch and glue method used polyester. (Epoxy makes it better)
For particular books on kayaks and polyester resin check on David Hazen's book on strip built canoes. This is an excellent book on strip building canoea and kayaks, and includes full size patterns for station frames for a few kayaks and several canoes. In fact, one of the few flaws with this book work is that it DOES apply to polyester resin, and has little info on epoxy, which has become much more popular since the book was published. If you can find the earlier editions of Gil Gilpatrick's books on strip building you will find info on using polyester. I have the 2nd edition and it covers both polyester and epoxy. In it he mentions that his 1st edition was devoted to polyester. The third edition is out now, I believe, but I havent seen it, so I have no idea what resin he is pushing now.
> 3) WhenI was mixing the resin with the hardner, it turned dark. Why is
> that? (The temperature in the room was 40F).
One of the nice things (?) about polyester is that you use one hardener, and you can adjust the amount to match your working temperature. That is also one of the problems: First you have to know the temperature before you mix anything. I have no idea why it turned dark, though. That doesn't sound right.
> Thanks
> Ben-B
You are welcome. Hope this helps
Paul G. Jacobson
This is an archived message from The Kayak Building Bulletin Board.
Guillemot Kayak - High
Performance Wooden Sea Kayak Designs
