3M High Strength Small Hole Repair for wood filler

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Aug 6, 2022
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Saw this product recommended on this site some time ago and bought a tub. I used it for the first time last night on a balsa fin. Thinned it some with water to make it brushable. The first side swelled and warped the fin badly. The second side unwarped it a little bit, but not enough. I clamped it between a couple chunks of plywood with a few layers of newsprint over the fin in a bench vise overnight and it seems to have dried nice and straight. But I'd like to not have to do that again.

Is anyone else using this? Do you thin it before brushing it on fins?

I've previously used the DAP Plastic Wood filler and thinned it without significant warping problems:

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I use the Elmers Carpenters Wood Filler, thin it with a few drops of water so I can brush it on, and I put it on both sides of the fin at the same time. I've done it that way on 2 rockets with the fins glued to the tubes already and had no problems with warping. If you do it on a bare fin you run the risk of the fin warping, if you try clamping the fin between 2 flat surfaces then how does the filler dry? Maybe you could clamp the fin between some thin pieces of wood so most of the fin area isn't blocked.
I didn't have very good luck with it and once I thinned it down a little bit, but I opened the tub back up and it had already started to harden, so I quit using it.