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Wood fillers etc.

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Senior Space Cadet

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I've tried a couple wood filling products and I'm not very impressed. They are gritty and don't spread that well. Someone suggested thinning them out with water, but the extra moisture warped my balsa fins. No one wants warped fins. Maybe there's a trick I don't know, but I don't think much of wood fillers.
Wood glue seems to work better, in my opinion, and probably adds strength to the wood too. I use a spatula or scraper to spread it.
I recently bought a product called Titebond Quick and Thick. Originally I was thinking I'd use it for construction, but it isn't as strong as wood glue. It does have better gap filling properties, so might actually be stronger in some instances. Being thicker than wood glue, it makes a really nice fillet. Since it's not as strong as wood glue, it might be best used as a second coat over a wood glue fillet. It seems to work really nicely for spreading over balsa before sanding. It doesn't do well with moisture, but I'm not sure that is much of an issue with rockets, unless you launch in the rain, and it would be under paint anyway. Dries clear. So far, it seems to be the best thing I've tried for filling balsa.
Don't remember exactly what I paid for the Quick and Thick. Probably about $5 for a large bottle, at Lowe's. I think it's worth experimenting with. Like I said, it makes a nice looking fillet, has good gap filling, doesn't run, and might make a really good wood filler for fins and it's fairly cheap.
 

Funkworks

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Saw dust mixed with wood glue is a common filler for full-sized carpentry projects. I’ve experimented a bit with balsa dust and wood glue for repairing a rocket nose and fins but not for structural fillets. I would think it can do a good job. Tuning the thickness is just a matter of selecting a dust-to-glue ratio.
 

dr wogz

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For fins, look at "papering". Creates a smooth surface, also adds strength!
You can also look at 'glassing', although that is a little more work for the same result (for LPR) (Even just a coat of epoxy.) Once applied, heat it up with a hair dryer to get a really smooth glassy finish! (The heat makes it runnier, and more flowy, also speeds up it's cure time, so be careful!)

As for fillets, epoxy is always my choice. Epoxy with a filler: micro balloons, West system fairing additive, balsa dust, even baby powder!
 

lakeroadster

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When using a product like Elmers Carpenter Wood Filler, thin it down with water until it's the consistency of tooth paste. Then fill both sides of the fins at once, that eliminates warping... more here: CWF

Papering, as Dr. Wogz discussed, has the added benefit of making the fins much stronger.... more here: Papered Fins

I personally prefer using basswood instead of balsa. The added weight is negligible, basswood is much stronger, and I've found there is no need for using wood fillers when using basswood. Just use a good primer like Rustoleum sander / filler, sand between coats, use 3 coats, and then finish paint... more here: Basswood
 

XrayLizard

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Today I found a new formula... lol
After so many evenings years ago with sig sanding sealer etc and the wood fillers. Etc

melamine laquer with “rottenstone” mixed in.
Apply liberal amount, scrape off with old cc, across (not with) the grain
Two, three coats.
Sand smooth... in no time, easy sanding... apply duplicolor hi build primer a few coats sand smooth and your ready to paint
It’s so easy even a model rocketeer can do it!
PS it’s a darkish gray. So there’s that.
 
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