I know this is in the archives, but it is spread across so many places and with such variation (contradiction) I wanted to consolidate answers.
What thickness balsa are you using and with what weight foam and glassing schedule?
I’m thinking 1lb foam with 1/16th" balsa with 2oz glass under and 4oz on the bottom and 6oz on the deck. Is that going to be more bullet proof than shop racks? Will this schedule work well for both shorties and longboards?
thanks
Heres what I’m doing:
Wood:
3/16" strips for the rails (they bend way better that 1/4" ). I use 4 strips
a skinny 1/8" for the deck skins (It allows a little more sanding room which I benifit from)
Glass:
6 oz on all 4 layers for a larger board
4 oz on all 4 layers for a shorter board
Benny had mentioned that he had talked to a long time balsa builder who said it was best to use the same weight glass for inside and outside. Being new to this I tend to error on the side of strength, but I feel I could get away with much lighter glassing schedules. I think that Bert has a lot of tricks up his sleeves which enable him to get away with the super light glass schedules.
1/16th’s good 1/8th will give more leeway which is nice but mixing on the same side is hard.
Also you don’t have to do the under glass
Double 4oz is strong enough and double 6oz is over kill if you don’t seal the EPS
I’d go 1/16 2 plus 6 deck and 1/8 2 plus 4 on bottom for lightness
change the 2’s to 4’s for more strength
with 1lb foam. i am doing the 1/16balsa 2oz under 4oz over bottom and 1/16 4oz under 4oz over deck and the seem strong as hell… longest i have made is 7ft…
one of my theories as to why i am doing 4oz under/over on the deck and not 2oz under 6 over is that i can use less resin because of the wet out table for the underlayer…
on my next personal board i am going to go even lighter… perhaps 2oz, 2oz, 2oz, 4oz…
I don’t know, Oneula, without the under glass its not really a sandwich… 
I went 6 oz under & over with a shy 1/4" balsa on my longboard. Been surfing it probably 3 times a week for 6 months now and not a ding, dent, or even scratch.
Went 4 oz under & over with a light 1/8" balsa on the fish and I dinged it (rail) the second time out and now its got (unrelated to the ding) a big delam bubble on the deck of the nose.
4 oz under & over with 1/8" d-cell and wood veneer on the 12’. Seems as solid as the 1/4" balsa strip longboard, but with less flex & return because d-cell just ain’t wood.
I’d stick with either heavier glass & at least 1/8" of balsa or lighter glass with nearly a 1/4" balsa schedule. Longboards especially. From what I’m learning about sandwich skin strength, the distance the two hard skins (the glass layers) are separated is directly related to the overall strength of the skin. The only reason not to separate them farther is that the core of the sandwich tends to be much heavier than the core of the board itself and you’re trying not to go nuts with weight.
All the above boards were 1# EPS, unsealed. Edit: None are vented and only the thin sandwich has ballooned. The longboard has spent 10 times more time locked inside my car as had the fish when it grew its tumor.
Now I’m working on another, also 1# EPS, but sealing everything. Using 1/8" d-cell and no wood. 1 pieces 1/2" d-cell on each rail. Its aimed at being a real Burger - fat round rails to try to separate the deck & bottom as much & as quickly as possible for the shearing action as related to flex that Bert kind of hints at. Wide, very blunt nose to parallel the rails. And then its got the Bob Miller glass treatment - bottom d-cell gets inside AND outside glass before the deck d-cell even goes on, so the underside of the deck d-cell is tied to the outside glass of the bottom. As its got no stringers, that should help greatly with strength.
Plus, when I mentioned that Bob told me to try that, Bert said he wouldn’t have said that much
But for 1# EPS, espeically on a longboard, I’d go WAAAAY stronger skin than 2 oz / 1/16th / 4 oz. If you have a 2# core, you’d probably be ok, but then your core won’t be flexing & shearing like it could otherwise.
I figure, with 1#, these boards are basically hollow. The foam is just there to aid construction of the skin, but once its trapped inside, its not doing much that plain old air wouldn’t do.
4EST
I have found that a total of 8mm works well,either go 4mm top and 4mm bottom or for a change of flex 3mm bottom 5mm deck.
You can mix and match all you like but if you go much thinner than 2.5mm to 3mm you lose the sandwich effect.
On the rails I now use 2 layers of 6.5mm
Glass bottom 3 oz both sides
deck 3 oz inside 2 x 3oz outside
This gives you a super strong durable board the last one i did weighed 1.8 kgs with fins
Think about this, the glass on the inside only has to hold on to the blank untill the outer skin is laminated
After that as long as there is enought separation between the glass skins it will hold the shape and the eps will just be doing the job of air
Mix up that with systems (there are quite a few ways of doing this) that tie the fins to the deck with a shear float and you have dynamic flex that you can control with the sandwich thickness
Hve fun Mike