The ideal way to join the stringers is by scarfing it on a 12-1 ratio but i was taught by mark just to sand down one vainer of ply on both sides and each side join the two pieces with glue than glass the sanded sections over on both sides but do not wrap the entire stringer with glass
BigE how did u machine your wood down from 6cm? i asked a shop to do mine 10 years ago i think i was charged a extra fee for re-sharpening the planer blades plus i had to drive from the West to East just to get the timber as it was the only shop that would plane it down