If you soak the balsa in water, it is bendable. Probably easiest to make a form to press the fins against, for uniformity. They need to be held in place for at least 24 hours. I seem to recall hearing that ammonia added to the water aids in bending.

I have used water and a heat gun to moisten a warped fin. Obviously an unsealed/unpainted fin. I misted the fin with a spray bottle, then heated the fin and bent it back straight. Be careful to avoid heating the root edge if the fin is already glued onto the rocket. You can soften the glue joint enough to debond the fin.

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Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp