Fillet Between Body Tubes

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That's fine. Add another bead to fill in the bubbled spots, or if it's strong enough already use some Bondo. It's lighter and easier.
Use white glue to fill gaps. Spread the glue along the fillet using a plastic spoon or finger. Let it dry. It may take 1 or 2 applications to fill the gaps.
Yellow glue like that is actually a good first layer, to soak all the way into the joint. But don't ever expect it to end up smooth (this is also standard behavior when filleting launch lugs); it will *always* bubble like that.

My approach is to start with a light fillet of yellow glue (not too much) and then follow up with Quick and Thick. Even the Q&T can bubble (although much less than the regular stuff), so a second small application may be necessary to achieve maximum smoothness.
I find, if I run a thin bead of glue into the joint and run a toothpick or kebab skewer (depending on the width of the gap) down the joint, twisting the toothpick as I go. It seems to push the glue down, into and onto the slope of the bevel between the tubes. Doing this doesn't eliminate the voids created by air trapped in the groove, under the glue, but seems to reduce it quite a bit. I'll add another layer or two on top of that until I'm satisfied. For adhesives, I mainly use Gorilla Wood Glue and either Elmer's White or TBII.
After a layer or two of yellow glue like you have, I'll fill with minwax wood filler to smooth it out.

On larger rockets, 5 or 10 min epoxy applied early to a horizontal fillet will settle into a nice smooth shape.
How would you get epoxy in there without making a mess? Wouldn’t epoxy be too thick for a syringe?
Yes, it would be too thick for a syringe. I tried it. Once.

I use a piece of scrap, or small popsicle stick to drip it into the crevice. That's why I say to apply it "early" before it starts to thicken. Have keep the fillet horizontal too. Once I've dripped it in there a fair amount, I'll run my littlest finger along the crevice, which pushes it further into the crevice, evens it out, and pushes some epoxy up the sides of the tubes a bit. Wipe off my finger. Then use a larger finger to run along again and smooth out the edges on the tubes into a nice smooth edge transition. Let it sit horizontal for a while. Keep an eye on the ends, especially for larger fillets. The epoxy is still fluid, and could sag or drip from the ends. Just smooth it back with your finger every few minutes as needed. After it dries, it usually just takes a little sanding to touch it up. If I'm sloppy, I might have to add a little wood filler on the edges to fair it out better.

But honestly, while your original picture might look a little ugly, it should be stronger than the cardboard tubes themselves. I've never had a glue joint break apart. The material around the glue maybe, but not the glue itself. So at this point, you're looking at a cosmetic problem. Fill it smooth with anything you like, sand and move on.
Not at all. I use epoxy in a syringe a lot.
I guess that depends upon the epoxy and the syringe. 😂

I tried some I got at a hobby store and it wasn't helpful. But I did use a couple of the JB weld epoxy self mixing syringes when putting some fillets on my King Kraken. (Couldn't get my littlest finger in there for all of them!) But those things are an expensive way to do epoxy, so I wouldn't do it regularly.


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