Carbon fiber tape. I have an old Pica Rapier, 40 size, that had a crack in the leading edge balsa. Got out my iron and heat gun, lifted the covering material and pulled it back far enough to access the crack. I then ca'ed a piece of carbon fiber tape about 2" by 4" right down on top of the leading edge sheeting. Then I ironed the covering back down. I don't think it took me all of 20 minutes. Because the carbon fiber is only .007" of an inch thick it's hardly noticeable. Use thin ca and sort of roll the cf tape onto the balsa while applying the ca. I would wear a glove because the ca gets a little warm when it kicks and a plastic baggy too to prevent it from sticking to your hand. When you are done it will be as smooth as the wood and extremely strong.