Hi, ive built solid balsa boards from 6-8mm x 100x1000mm planks. Its cheap and available in Europe from " hobby" and radio controlled airplane builder hobby supply.
Problem is balsa is super soft and sucks water like a sponge, and disintegrates if it get wet. You must laminate over it to protect and get out of water immediately if you crack or ding laminate.
Using the 18mm paulownia from Bauhaus, its much more work, but not too bad. I cut up 18x600x1200 plank into 4 pieces, 18x300x1100. Glued 3 of them into 54x300x1100 block. Sliced off 3mm strips from that on table saw. Edge glued 3x54x1100 strips with CA glue dots. Used 2 skins with layered rails with carbon between skins and eps.
You can see my pics in previous post. Thanks to other builder for the method!
It weighs about 2,2kg now.
Making test pieces of different wax, oil, varnish and one component pu varnish to find best finish without lamination on outside. Testing ongoing, have one more option to test.
One already looks like it will work. BUT, i can say already that paulownia is not that hard. I can put groove in it with fingernail fairly easily. It doesn't break the varnish layer, but the wood gets a groove. The boards will need to be treated with care. You cant just throw them in your car or drag them around on the beach.
Only way around this is to use a glass laminate, but that would be a real shame, add 500g and bring back the boring part of board building.
I think with a little care and some re- varnish once a year ( hej its wood!!) They will last a long time and get a beautiful patina.
Next build i will use glass fiber, not carbon as it is overkill. I hade 1/4 of the plank, which cost 36€, left over, plus quite a few extra strips. So entire build is really cheap in materials. 1m2 glass, maybe 0,5 kg epoxy with waste. Finish varnish 10€. Bigheads 4x 3€.
Another way to do this would be to completely eliminate the paulownia deck skin, and put 2x glass on top of foam core and full cork deck on top of that. Would have to use slightly higher density foam core though.
CNC is running now, so plan on milling parts and modify method to reduce need for so many rail parts. Might even be able to mill entire bottom and rails as one part, and only skin deck.