I've been in rockets for a long time and the conventional wisdom was always for the grain direction of the balsa to be parallel to the leading edge of the fin. I never questioned this until now. I'm building a HiFlyer XL (because I found a pack of D12-7 in my gear!). The HiFlyer XL has long narrow triangular fins so the fin grain ends up almost parallel to the fin root. Well I'm not a structural engineer for real (a rocket scientist just for fun) and since the fin is trying to cantilever from the fin root, the fin grain should be more perpendicular to the fin root and not parallel to the leading edge.

Balsa is pretty weak in bending perpendicular to the grain direction, and since I couldn't re-cut the fins I came up with another idea. I papered the fins to give more strength in bending in that direction.

Papering is a whole nother subject. I've done it before and I've watched people do it on youtube. I put wood glue on the fins with a small paintbrush, then stuck on the paper. The paper sucks moisture out of the glue and wants to swell and wrinkle. I rubbed down some of the wrinkles with a putty knife and then flattened the fins with a heavy piece of tile. The few remaining wrinkles I will cover up with fill n finish. I had the idea afterwards that if I had somehow moistened the paper first I would not have had the wrinkle problem. Maybe hold the paper in front of a steam iron for a few seconds. If I do papering again I might try this.