Re: stringer material, again
I suppose part of the answer depends on how long the Stringer is going to be? In my resto last year I had to replace ALL the wood. For the most part I used plywood that I picked up from Lowe's/HD, ranging in thickness from 3/4", 5/8" down to 1/2". Some will advocate using only marine plywood. Problem is that the main big box hardware places don't stock it any longer and can't order it for you. Then it's way more expensive. Allegedly, the adhesive used in marine ply is different to regular plywood. The main difference is that Marine plywood is made up of many more layers than regular plywood and their are no voids in the timber either. Plywood because is is made up of layers with the grain going in opposite directions on each layer, has greater rigidity/flex and strength than regular timber, which has a grain which goes in only one direction.
If your stringers are more than 8' long, and decide to use plywood, then you have to somehow join them together. I tried that and it failed miserably. Instead I used regular timber that was the length I needed 12'.
TBH, so long as you seal the timber with either epoxy or polyester resin (I used polyester resin throughout), then encapsulate with fiber glass, you should be fine.
I cannot comment on the composites that are around, like Starboard, Kingboard, etc. Main thing about them is they are expensive and in some cases do not have the structural strength that you need for stringers.
I am sure an 'expert' will be along shortly.
