I made some home-made casein glue using this online recipe. It was incredibly easy. I clamped two pieces of redwood – roughly 1’x2″x2″ – together with some of this concoction and let it cure overnight.
In the morning, I found two pieces of redwood that were very strongly glued together. I was unable to pull them apart with my hands. I threw them on the cement ten times with all my might – they remained intact. I threw them against the sharp corner of a concrete step ten times with all my might – not only were they intact, there was absolutely no sign of weakening.
I submerged them in water overnight. In the morning I found two pieces of redwood that could be pried apart with my hands with fairly little effort.
I’ve read that casein glue is water resistant. It may be possible, considering the design of these surfboards where the glue has no immediate exposure to water, to use this glue. The potential issue would come from dings and tiny cracks that let water through to the glue. This may not be enough of an issue considering the high amount of surface area where there will be glue. Trial and error are the only ways to truly test this.
Another preoccupation is that the glue was very clumpy and, because of this, didn’t allow the two pieces of redwood to clamp together as closely as I’d like. This could possibly be fixed by using more water during the final mixing stage. If anyone has experience making casein glue, comment below, I’d appreciate your insight.