This is how I did my 4 piece collapsible…
Build a wooden frame, covered it with cardboard and filled it with can foam
https://www.woodboardforum.com/filedata/fetch?id=5628&d=1517686094
This was the result:
https://www.woodboardforum.com/filedata/fetch?id=5638&d=1517771748
and after some sanding, it sands much better than eps…:
https://www.woodboardforum.com/filedata/fetch?id=5656&d=1517900222
I the glued wooden veneer directly onto the “blank”, during the vaccuming of the veneer some slight dents appeared, but it finally is working.
My next project was a collapsible bodyboard for my wife. Here I glued on the Deck and bottom using 4mm balsa first and filled the frame with can foam too, prior to build the rails. the results are much better than with the cardboard, but unfortunately I only have pics of the final board:
https://www.woodboardforum.com/forum/design/workshop-tool-ideas/6169-new-bodyboard
(Watch the interesting connection between the collpsible parts; it workd out pretty well, the connection is solid and releaseable the only thing is you have to protect the connectors from saltwater. I had some saltwater contact, but finally nothing seriously happened, but I protected additionally with a lot of oil and cleaned everything after use)
My upcoming board for autumn will be constructed according to this method. wooden frame, deck and bottom planked, the filled with can foam. The expansion forces are not a problem at all, but this depends on the used foam. mine is very light, about 15g per litre final weight and its supporting deck and bottom well, and I leave the rails open for expansion. some areas where the foam is denser or has bubbles, but it works pretty well. since it will be encased in wood completely I do not see a danger from UV rays.
For me, this method is quite perfect, because you finally evitate to shape whatever kind of foam, no mexx at all, just cutting at the rails side, ready. the waterabsorbancy of the used foam is 0,3% but its better that everything is tight. My 4 piece collapsible sucked half a cup of water through a pinhole. Nothing seriously, but it demonstrated that under sucking pressure (if hot board gets into cold water, the foam is not really watertight, it must have at least some open cells. the shape comes from the wooden frame, which is easily done, especially if using balsa. the balsa deck will additionalyy veneered, here its a little bit tricky, because I’m using vaccuum and you need to have the pressure high enough to glue the veneer, but light enough to not deform the deck (and foam below). Otherwise you will have some dents between the ribs. If you look closely to the final bodyboard you will see what i mean:
https://www.woodboardforum.com/filedata/fetch?id=6211&d=1526208947 (a concave on deck from right to left was on purpose, to get more hold in the water…)
This is the “Surfdude/Olddudes” way of building his upcoming boards… Hope the next board will be my best one built ever. it will not be collapsible and should be at 8’3’’ length not heavier than 10lbs…