Plywood gets it's strength from the verneers that it is made of. Plywood is the strongest if the verneers are all about the same thickness.
Most luan is made with a thick core and paper thin outer verneers. This stuff is meant to lay under sheet flooring. It isn't intended for any use that requires strength. Most builders that use it cover it inside and out with epoxy and fiberglass which makes the boat twice as heavy as it should be. It also eats up any savings that were gotten by buying luan in the first place. Cheap luan also doesn't bent very well and is inclined to tear at the points where the wire ties are located.
There supposedly is some luan that is made from equal thickness verneers but I have never found any in the bigbox stores I have visited.
And there is more bad news. Some bigbox stores now sell luan underlayment that is made with "water resistant" glue. Boatbuilding plywood should have "waterproof" glue which is a different and much better animal.
A sheet of 6mm high quality okoume boatbuilding plywood weighs about 20 lbs. per 8x6 panel.
4mm weighs about 12 lbs. per 8x6. 3mm about 8 lbs.
I use West Systems pumps and burp them before I start pumping. Haven't had any trouble after about 5 years of use.
I finish my boats with varnish because any spots where water gets below the finish will turn black and will be easily seen and corrected.
Building light is difficult even for the experienced. It seems that any impulse to make things "better" always adds weight. The big weight culprit is epoxy. It weighs about as much as water and should be used sparingly to keep things light.
Hope this helps.
Charlie

: Having not mastered the art of the short post, here is a summary of these
: questions: 1. Is 5.2mm lauan too heavy to use for an 18' boat?
: 2. What weight cloth can I get away with if doing both sides hull and deck
: with 9oz taped interior seams and reinforcement strips.
: 3. Do Raka's epoxy pumps work well?
: 4. Will the thick and presumably soft core ply of the lauan dictate thicker
: cloth for puncture and ding resistance? Will it make a weak composite
: sandwich?
: 5. Where do you get copper wire for stitching locally? Or online.

: Went down to Home Depot today and bought some 5.2mm lauan 4x8 panels. They
: had 3mm also, but this was in 3x7 panels, and looked crappy with voids.
: Thick stuff has no voids visible with light (but they may be discovered in
: cutting), thick core, really thin outer veneers light and "OK"
: looking on one side, dark and rougher on the other. It feels pretty heavy,
: but I've never held a full 4x8 of 4mm Okoume before so I can't judge. Ross
: Liedy recorded using 5mm lauan on his version of the Sea Kayaker S&G. He
: also commented that perhaps battleship gray paint was appropriate but I
: got the idea somehow that his boat came out around 50#, which I would
: consider acceptable--but I think I gleaned that from one photo caption
: which said 45# for the hull and deck and it wasn't finished.
: Ross, you around? How much did your boat weigh?

: If you don't think it's too heavy, what about in terms of bending? I *think*
: that if I build the boat with the paneled deck as designed, it should make
: those bends. Maybe some trouble with the bow and stern joins. But will I
: wind up with a 65# + boat? The four full sheets of wood are too cumbersome
: to hold at once, but just lifting the edge on four sheets is heavy. I'm
: not worried about it being a little bit troublesome to fit and stitch, if
: it works in the end, my primary concern is that it may just be too heavy.

: If it is deemed usable then what does the collective say about maybe using
: all 4oz FG cloth instead of 6oz on the hull and deck with 4oz inside? I
: don't have a good feel for weights of cloth. The stuff in my previous kit
: was heavier for the hull than the deck. I *think* that was 4 and 6 ounce.
: Also, what about the structure of the composite sandwich? Aren't thicker
: softer cores more susceptible to puncture or dents and dings possibly
: compromising the skin? Should that be offset with heavier glass? How much
: weight does a glass job add anyway, roughly speaking? Is two layers of
: lighter cloth stronger than one layer of thicker? My understanding is that
: most of the weight addition in glassing comes from epoxy. What do the
: ounce designations on the cloth mean? Is it 4oz a yard? Or 4 oz per sq.
: ft?

: I'm planning to go to Raka for epoxy and cloth as many folks seem to be
: pleased with them and I have not found any cheaper prices. Speaking of
: which, do Raka's pumps work well or should I maybe think about using
: syringes and measuring cups?

: One more question: Where in the heck do you buy copper wire or other wire
: suitable for all this foolishness?

: Thanks!
: TD