Good info Guy!
Last night I tried the following, motivated by the thread I had just read.
I took a couple small scraps ( 3" X 24") of 5.2mm luan panel and cut a similar size of 1.5" thick DOW blueboard (Extruded Polystyrene or XPS). I moistened one face of each plywood with a damp rag. I lightlty ran some 120 grit sand paper once over both sides of the blueboard to roughen the surface and I wiped the faces off with the damp rag. I buttered both sides of the blueboard with a thin layer of Gorilla Glue (polyurethane based) and made a sandwich between the damp faces of luan. I clamped and went to bed.
This morning I unclamped and wow! That son of a gun is strong. I span about 18" between two chairs and stand on it with one foot. Hardly any deflection. I don't know how it could bond much better,
Now.... am I missing something or is it really that easy to build a composite panel? I had been experimenting with pouring the foam but this seems equally as strong, and less messy. The glue is expensive but maybe it can be purchased in larger quantities cheaper.
I will try the same experiment with beadboard (white expanded polystyrene - cheaper) and see how well it works. Also HD had a pink XPS 2" foam board (FomulaR) for less than half the blueboard price so I may try that too.
I figure a 4' X 8' panel would cost $22 for two sheets luan, $ 12 for the foam board, and $8 for glue. Thats $ 42 per panel.
Still thinking about Guy's swimming pool clamping idea. Water weighs 62 lbs, per cubic foot so that means a 1 inch square column of water 1 foot high exerts (62/144=0.43) 0.43 psi pressure. For even 2 psi it would take 5 feet of water. Assuming you built a rectangular "pool" overlapping the 4 x 8 panel sides by a foot in all directions, that would be a 6 X 10 pool 5 feet deep or 300 cu feet. That is 2244 gallons of water. It would take a long time to fill that with a home water tap. So we need another plan. Now Honda makes a 5.5 HP gas powered pump with a 3" hose and a transfer rate of 275 gallons per minute that you can probably rent. That is just over eight minutes to "load" the water. I guess you would need another "pool" to hold the water as it has to be ready to go when you need it. Maybe you could make one panel with a first pool and then transfer the water to the other pool to make another when the first is done. My neighbors are going to love this!
I think vacuum bagging may be a better solution. I have been told a good shop vac bas deliver 3 psi of suction, that's the about same pressure as a 7 foot column ( "pool") of water. Now...but if we could fill the pool with mercury... no just kidding.
Where am I going with this composite panel stuff? How about a Weekender type design made of composite panels that are joined by both edge glueing and using epoxy fiberglass on the inside and outside of the joints.
I think composite panels make sense as the side panels of a tear too.
Any comments?