If the flap is straight to begin with, and if you use a product that does not shrink then it should be straight when its dry. hence epoxy, Ok so lets see what the problem really is, the problem is that we want stiffer flaps with no weight. Ok so solutions, glass fiber and epoxy. can be very heavy. and likely overkill. How about carbon mat, or veil, well since the fibers are non directional, you gain only a small amount of strenght from them. especially if you use dope to adhere it you wont gain much more than silkspan though you may think you do because its "carbon fiber" . SO you are adding a fair amount of weight with marginal strenght increase. To maximize the strength, you need to align the strength of the material with the lines of stress. So, then we have to decide what are we trying to accomplish. well longitudinal rigidity isnt really all that important since we use enough hinges to keep it from flexing tha tway. In fact, i challange that making the flap to rigid from tip to root is a bad thing, especially if any flex is present in your wing, which I would imagine is likely in most wings under high load. Ok so we need to eliiminate the torsional twist.
this means that the fibers in the matrix to most maximize their strenght to weight ration shouls be on a diagonal bias. fibers aligned any way but diagonally are not helping to add to our solution they are only addign weight, and soaking up resin . so, align your cloth on a bias, use .75 oz cloth and only enough resin to bond it effectifly. the optimal ratio of cloth to resin is one to one. This means for every square yard of .75 oz cloth, there should be .74 oz of resin. Now in a hand layed matrix this is virtually impossible to obtain so we are going to have a bit of excess weight,but not much. there is another option. One thing that is lacking with Nitrate or dope, is a good matrix bond and the contiual shrinkage that occurs with dope. Howa bout if we used another more easily controlled catalyzed product. One thing I have been experimenting with is using urethane clear as a bonding matrix. It has some very good properties. It is catalyzed and does not shrink further once cured. it si easy to flow onto a surface. Might be worth considering. It will eliminate the problem of uneven shrinkage as well since it doesnt shrink. You may actually consider using it with silkspan or CF veil. Just remember that you wont get as much strenght with veil as you would with FG cloth. but it will also be potentially lighter.