[JohnB]

My theory on cored hatches is as follows.

Polyglass is not porious, but over time, moisture will migrate through it. I own a moisture meter, and I haven't found a piece of coring yet, more than 5 years old, that has polyglass on it, that isn't at least "moist", if not wet. Especially if there is a crack or holes in it. Also, if you look at the bottom of the hatches, most are not completly sealed up with glass, there are holes/gaps. Also, many times the water that is on top of the hatches is fresh water/rain water. Many boats when sitting still at the dock or on the trailer, puddle fresh water on their decks. Fresh water causes rot much faster than salt water.

Once the wood gets damp, problems start. There was a lot of "non-marine" plywood put into boats over the years, and this stuff has water based glue holding it together. I think over time, water gets to the glue in the marine plywood too, it just takes a lot longer. In either case, moisture is step one in the failure. If you live up north, and a boat goes through a freeze/thaw cycle, this can excellerate the process too. Wet transom boats up north that have been freeze/thawed have the famous hollow sound since the glass has been pushed out away from the core.

Whether a stress crack or scew, there almost always is a path for water to get in. Once the "break" happens, the penetration of polyglass is so thin into the wood, that the water goes right in, and starts to migrate between the wood and glass gap, and then into the wood. When you use epoxy, it fills all the air gaps the water would use, and goes deeply into the wood. Even if you get a separation between the skin and the core, if you have precoated the coring, water can't get into it. At this point, I think I like wood better than foam, since epoxy seems to go deeper into wood than it does closed core foam. You loose a little on the weight side, but the lower cost and compression strength of high quaility marine plywood offsets it. If your using polyglass with foam coring, the good news is the foam coring is closed cell, and won't wick up water. As for the cost difference between epoxy and glass, the cost per gallon is about 2x, but you use less epoxy to get the same strength. I use epoxy for all coring (transom/hatches), and to stitch or connect pieces. I still use polyglass to build caps, hatches, livewells, and fish boxes.

I am redoing my livewell hatch this week. It was all original, plywood coring, and will post some pictures.