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The bond between polyester and wood deteriorates over time. It is usually pretty good at first but over time, slight amounts of moisture absorbsion cause the polyester resin to let go of the wood.
The reason for this is that polyester resin is, believe it or not, somewhat water soluable. As wood is very capable of moisture absorbsion, the interface between the wood and polyester is subject failure over time. This is a common problem in sail & power boats built in the 60s and 70s which used plywood as a core material with polyester laminates on the outside. Most, if not all, of these decks which were fine in there early years are now delaminated. Repairs are very costly.
For more details on the water/polyester phenom as it relates to immersed polyester laminates, check out http://www.zahnisers.com/repair/blister/blister1.htm
Epoxy is far less water soluable as well as having stronger physical characteristic. Vinylester approach the characteristics of epoxies but as they are not yet commonly available in small quantities and are nearly as expensive as epoxies, it's hard to see the point.
Of course, if you can keep the joint dry, well, polyester might work for the long run as well as the short run. Keeping things in boats dry however........ On the other hand, if your into prototypes and don't care how long they last and saving $30-$40 is paramount, poly might just do the trick in the short run.
Craig
> Hi,
> I am playing around with an inexpensive polyester epoxy (Bondo). 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.
> Here are my questions: 1) how strong is strong enough?. Is the polyester
> strong enough for kayaks.
> 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?
> 3) WhenI was mixing the resin with the hardner, it turned dark. Why is
> that? (The temperature in the room was 40F).
> Thanks
> Ben-B
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Guillemot Kayak - High
Performance Wooden Sea Kayak Designs
