Getting Good Fillets?

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MALBAR 70

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Ok guys, I have a confession to make,....I don't know how to get good, smooth fillets, without tons of air bubbles.
I use either Titebond II or plain old Elmers White glue, and yes, I have used TB Quick and Thick without much success. The only thing I can get good fillets with is epoxy and that's a little heavy for LPR.
I've tried the wet finger,Q-Tips, coffee stirrers and some other methods as well. I just can't seem to get nice fillets without 100 layers of glue and an application of Glazing and Spot Putty to fill in the abundant voids.

So, what's the secret?
I'm willing to entertain any and all tips, tricks and techniques.
Thanks.
 

mbeels

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I forget who it was, but someone here was getting beautiful (and light) fillets on LPR with some kind of caulk. I haven't tried it, but his photos looked really good.
 

Karl

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I use Titebond II for LPR fillets; I think everyone has their own method for doing them but my personal one is ;

Mask around the area you want the fillet. I square the fillet off at the bottom of the fin and taper it to meet the tip at the front. I do two opposing fillets at a time with the rocket held horizontal and as level as possible.

Run a bead of glue into the masked area and form a uniform shape by using one of these.

Watch for any air-bubbles, these can be popped with a toothpick.

Leave the glue to rest up for 5-10 mins, it depends on the temp in your build space. The tape needs to be removed at the stage where the glue is firm enough not to run but soft enough to self-level and remove any sharp edges left by the tape.

If some bubbles have appeared once the glue is cured, I use another thin layer of glue just to fill them in.

I personally leave the fillets to cure overnight before starting the next two.

Takes a bit of practice to get them looking half decent but I think well-made fillets make a rocket really stand out.

Good luck :)
 

neil_w

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Ok guys, I have a confession to make,....I don't know how to get good, smooth fillets, without tons of air bubbles.
I use either Titebond II or plain old Elmers White glue, and yes, I have used TB Quick and Thick without much success.
Did you try my pseudo-tutorial for using Quick and Thick? https://www.rocketryforum.com/threads/doing-fillets-with-titebond-quick-and-thick.148632/ If it still didn't work for you, I'd be interested to hear exactly what happened.

Regular TBII or white glue are almost always gonna give you bubbles, unless you make them *really* thin.

@BSNW 's use of construction adhesive for fillets is certainly intriguing.
 

MALBAR 70

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Did you try my pseudo-tutorial for using Quick and Thick? https://www.rocketryforum.com/threads/doing-fillets-with-titebond-quick-and-thick.148632/ If it still didn't work for you, I'd be interested to hear exactly what happened.

Thanks! I remember reading that a while ago but couldn't remember where it was. Very helpful.

Regular TBII or white glue are almost always gonna give you bubbles, unless you make them *really* thin.

Most of the time I can get semi-bubble free fillets on fin to body tube joints. The ones that give me the most problems are tube to tube joints. ( I should have mentioned this before)

@BSNW 's use of construction adhesive for fillets is certainly intriguing.
 

neil_w

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Most of the time I can get semi-bubble free fillets on fin to body tube joints. The ones that give me the most problems are tube to tube joints. ( I should have mentioned this before)
Ah yes, that's inevitable: launch lugs and tube fins and the like. Even Quick and Thick can bubble a bit under such circumstances, although much less than the regular stuff.

Nowadays my general approach is first to apply a thin (aka quick-drying) TBII fillet to flow in there and seal the joint. I don't care if it bubbles. Then I apply Q&T on top of it. Success rate is pretty high for me at this point using aforementioned techniques.
 

afadeev

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I forget who it was, but someone here was getting beautiful (and light) fillets on LPR with some kind of caulk. I haven't tried it, but his photos looked really good.
Caulk is as heavy, if not heavier than Epoxy. Caulk is significantly less strong, with a tendency to crack under lateral loads.
If strength and/or perfect fillets are required, go with Epoxy, every time!

If weight and "good enough" will suffice, TB-II + TB Q&T on top work really well.
I am a fan of TB-II just because it's easier to work with, non-toxic and non-allergenic (unlike Epoxy), and is generally quicker and easier to work with since no weighting of ingredients or mixing is required.

I've never needed large fin-fillets on low-power rockets, so the subject of weight rarely comes into consideration.
For the sake of simplicity, TB II+Q&T wins, almost every time.
Bubbles get covered with a few layers of paint.

The exception to the above is when larger fin-fillets are required for strength reasons (e.g.: longer fins on Strikefighter or Saturn-V).

a
 

Antares JS

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Just to throw it out there, I swear by Aleene's Tacky Glue for low power. Just apply a bead into the fin joint and smooth with a finger. I don't recall ever having a problem with it bubbling.
 

mbeels

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Caulk is as heavy, if not heavier than Epoxy. Caulk is significantly less strong, with a tendency to crack under lateral loads.
If strength and/or perfect fillets are required, go with Epoxy, every time!
I took a look back at the thread, and it is actually a construction adhesive. It must be much lighter than epoxy, epoxy is far too heavy for any LPR build. I don't generally do much for fillets in LPR (because strength is not required), but this looks like a nice way to get a good fillet appearance, without as much weight.

But the TB Q&T looks like a good approach for LPR as well.
 

afadeev

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epoxy is far too heavy for any LPR build.
I have to disagree with you here.
All my models get porky not from fin fillets (epoxy or otherwise), but from multiple layers of paint. Basically, making the rocket look good makes it heavy.
Papering balsa also adds way more weight then (thin, not fugly) epoxy fillets.

But the TB Q&T looks like a good approach for LPR as well.
Amen to that.
 

neil_w

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I have to disagree with you here.
All my models get porky not from fin fillets (epoxy or otherwise), but from multiple layers of paint. Basically, making the rocket look good makes it heavy.
To be fair, paint weight is a bit more distributed across the rocket, whereas fillet weight is concentrated in the rear (for the most part).
Papering balsa also adds way more weight then (thin, not fugly) epoxy fillets.
That is absolutely true. Worth it to me, but definitely must be taken into account w/regard to CG location.
 

mbeels

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Take a good sharp filleting knife, and cut along the bone.......
Oh, wait........
Ok, first Doorknobs, and now fillets. Should we be concerned, kuirin? 😆
 

Old School Doug

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I've used this cheat to fill in the fillet bubbles/pits. After filleting in the normal fashion with TBII (generally 2 passes for strength), I'll take the same thinned Elmer's wood filler I used to fill the BT spirals and apply a thin layer to the fillet, smoothing it in with a finger. After it has dried fully I use a damp (not wet) sponge to wipe off the excess and smooth out any thickness inconsistencies. Prime and sand takes care of any minor remaining flaws.
 

Bluegrass Rocket

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I’ve been using Elmers Xtreme white glue and it has the pointy cap. It seems better than regular Elmers school glue or glue all. The main thing to always do, is store the glue nozzle down. This gives any air bubbles a chance to raise away from the nozzle. When you open it the glue will be flowing so be ready. Run the point along the fin/tube edge leaving a line of glue. Use the outside edge of your pinky finger to rub against the joint removing a bunch of the glue. This should leave a smooth fillet of glue with no bubbles. Use some scrap balsa on scrap tubing to practice. The procedure can be repeated after glue dries.
 
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