HI like to add - when parts are marked and fit twice i use thick CA and 5 minute epoxy to join the wing center without a problem as the fit is carefully planned before any glue is appliedi will change any wing joiner in a 60-120 kit to 1/4 inch 5 plyplywood at the spar and increase the LE joiner to 5 ply also (strong stuff )i then join the wing center and SHEET OVER the center four ribs to increase the center sections strengthso i have no center seamthis is an old U control building technique-i have not used any cloth at the wing center for 20 years of buiding this way i do not like the way film covering hold and looks over epoxy filled cloth-i have not had a wing center fail on me in that same 20 years again my .02TONY