Fill n' Finish and balsa fin warping

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Well-Known Member
Jan 19, 2009
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When I use fill n' finish on balsa fins, they warp. Is there a secret to this technique?

I typically mix my wood filler to about the consistency of pancake batter (maybe a little thinner), and apply w/a brush. I've tried applying the filler both before and after attaching the fins to the airframe. There seems to be less warpage when applied after attachment; however, I would be much happier w/ no warpage.
i get warping when i use a thick dope sanding sealer when i do one side only, - equal amounts of sealer on both sides at the same time - no warping
I thin out my wood filler to a similar consistency, maybe a bit thicker. I don't end up with warped fins though. How thick do you put it on?
do both sides, and don't use too much water.

If you're doing 1/16" fins, "pancake batter" may be a bit too thin. Go for something between that and "peanut butter" consistency. You may not be able to use a brush; instead, rub it in with your fingers (protected by a glove or balloon).

Note that with the new "Stainable" filler (has bigger, harder microspheres, and annoying "real wood fibers") you'll almost have to use more water. grr.
From the sound of things, I think that I am thinning the filler too much. Do you finish the fins before or after they are attached to the airframs?
Any water-based filler could warp the fins due to the different absorption rates of the wood grain. Do both sides, then lay them between sheets of wax paper, under a flat board (or similar) and weight with heavy objects like books, a stack of ceramic tile, etc. They'll dry flat then.

Dope-based sealers shrink when they dry, so only doing one side at a time will 'pull' that side of the fin smaller than the other one. Again, do both sides and let dry on wax paper.
From the sound of things, I think that I am thinning the filler too much. Do you finish the fins before or after they are attached to the airframs?

I put them on the rocket before filling them in. I feel that I get better adhesion since the glue is in contact with only wood. If they're attached later then the glue would be in contact with wood and filler. However, you could always tape off the areas of the fin that will be in contact with the body tube or the fillets and then fill it in before attaching it.
From the sound of things, I think that I am thinning the filler too much. Do you finish the fins before or after they are attached to the airframs?

I've done both. And really, even if I fill them off the rocket, I don't use primer until after they go on the rocket, so therefore the final filling (since I use a spot-filling primer) is always done on the rocket.
i mix mine with white glue instead of water. use a putty knife to put it on and let dry then sand and repeat. this works really well if theirs large dents and holes. like my asp v2 the nise cone had a dent in it and the tip broke off the glue fille mix worked really well.
I have experienced warping as well when doing the same thing. I've learned that if I weight down the fins after applying the filler (using 3 or 4 testors paint bottles works well), I can stop the warping. Sometimes the bottles leave small rings in the filler, but these sand away when finishing the fins.
I skip the filler and just paper them. The only time that I use wood filler is, ironically, on paper tubes. When I don't paper them, I use dope sealer and balsa fillercoat. Dope does have a reputation for causing warping, but whenever I do one side, I always do the other side anyway just so that I don't have to wait. I guess that prevents the dope from causing them to warp, but I never realized it until now. I fill and seal the fins before I bond them to the airframe. It's much more of a hassle to try to do it afterward. I have never had any bonding problems with pre-filled, pre-sealed fins. I don't apply primer and paint until after they are on, though.

On most wood fins, I use light weight, quick dry spackle, but not on areas where glue needs to adhere well. It drys quick (4-6 hrs on the Gulf coast), sands super easy and paints nicely. I keep balsa fins between wax paper (or something else to prevent the spackle from sticking to things) and pressed between two flat surfaces to prevent warping. Of course, filling is done after shaping and before putting them on the body.
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I have stopped using the stuff except for body tubes and minor repairs. I too like to paper the fins. Not only is it easier, it makes the fins stronger.