> What's the problem with polyester? Too much permeability, poor
> bond, or what?

Yup.

Polyester resin and fiberglass cloth have been used to cover wooden boat hulls for decades. Some of those hulls are now showing problems. Here is my take on things: According to my sources, the catalogs of Glen-L and Clark Craft, as the polyester resin hardens it releases small amounts of gas from the solvents and volatile materials in the mixture. As these things migrate through the resin they leave behind microscopic paths which are far too tiny for molecules of water to penetrate. Water vapor, however, can get through, and over time enough water vapor will penetrate to permit mold, and rot to grow on the coated wood. These problems are most likely to occur on boats that are constantly in the water. Small craft ( rowboats, canoes, kayaks) that are removed from the water each day shouldn't have this kind of problem. If they did, it would be years before anything happened. Owners of larger boats just assume it is a cost of maintenance, and every decade or so they sand off all the fiberglass and resin, and recover the hull. This practice seems to follow the philosophy of: ``A boat is a hole in the water that you throw money into.''

Friends have advised against using polyester resin on red cedar because of the sap (resins) in the wood which adversely affect the bond. There is a work-around procedure which is suggested in Hazelton's book on strip building canoes and kayaks. He suggests using a wood sealer as a primer coat before using polyester resin on red cedar. White cedar and pine are apparently more amenable to being used with polyester than red cedar is. Epoxy works well on all of them.

As I understand it, the polyester sits on the surface of the wood, while the epoxy soaks in as much as 1/8 th inch under some circumstances. It is that soaking action which creates the boundary layer that seals out the moisture. Since the epoxy cures by polymerization, with almost no release of evaporating solvents or other volatile gasses, there are no paths for water vapor to follow. The wood gets sealed, and stays sealed.

Glen-L and Clark Craft have more info on this in their catalogs. You might check there, or with other references on fiberglass and resinss for more scientific data. But, to boil it down to the essentials: Polyester is nice, but epoxy is the miracle goop that laminates, glues and seals the wood. Hope this helps. Paul Jacobson