Newbie question about double glueing

Am I understanding correctly to apply glue to both fin and body, let it dry, then glue the two dried glue surfaces together? And this is done with white or carpenter's glue?

If you are gluing balsa fins to a paper cardboard tube you want to glue them together the first go round so the glue bonds and penetrates at the same time. Apply a generous bead of glue to the root edge of each fin and attach to the tube. I usually smooth out the joint with my finger and let dry. Wood glue shrinks so you will want to come back and run another bead of glue on each side (fillet) for strength and aesthetics. I use carpenters wood glue, specifically Titebond II because it is water resistant after it dries. There are many to choose from. I do not use white glue for attaching fins. YMMV.

a thin coat to each surface, let dry, then a second thin coat to the fin root, gently(but firmly) press the fin in place and hold for approx. 30 seconds, correct. white or yellow glue, also correct. takes a little practice but, once you get the hang of it you'll wonder why some folks are so enamored of using super glue .
Rex

I prefer yellow Carpenter's Wood glue. It sets and dries faster than white glue.

tomrcast said:

Am I understanding correctly to apply glue to both fin and body, let it dry, then glue the two dried glue surfaces together? And this is done with white or carpenter's glue?

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I think I can speak for a lot of TRF'ers who, like me, first heard about the double glue method by reading the "bible" - The Handbook of Model Rocketry by G. Harry Stine. A thin, even film on both surfaces, wait a minute or two, a bead on the fin root, press in place, done. You can do all your fins at close to the speed of CA (super glue) with any white or yellow glue. Try it !

What Rex said...

As a teenager growing up, I first read about "double glue joints" in G. Harry Stine's "Handbook of Model Rocketry". Being young and stupid, I thought to myself, "that's a silly way of doing it-- it would take FOREVER waiting on the first layer of glue to dry and then having to apply MORE glue and waiting for that to dry... nuts! I'll just slap the fin to the tube with a thick slobbery layer of yellow wood glue for strength!"

Of course, since I never tried it, I didn't know what I was missing... Decades later as a BAR, I tried the double glue joint once, and was INSTANTLY SOLD on the technique! I apply a thin layer of yellow carpenters wood glue to the fin root and the fin line along which the fin will be glued (after lightly sanding the tube with 220 grit sandpaper to increase adhesion by roughing up the glassine coating). Let that dry for about 20-30 minutes-- doesn't have to be completely dry. Most of the glue and the moisture from it will soak into the balsa and paper tube and it will "tack up" within minutes-- heck in an hour under most conditions it'd be completely dry, but you don't really need it to be (I don't think it will hurt anything if it dries, but very very tacky is good enough (glue thick and very sticky).

To hold the fins and tube while the applied glue dries, I take an old egg carton from the refrigerator and rescue it from the trash can, invert it, and cut slits across the egg cups (foam egg cartons work best) with a sharp hobby knife, cutting across the egg cups perpendicular to the long axis of the carton. This creates "slits" which you can stick the tip edges of the fins into to hold the leading edges up and horizontal while the glue dries on the root edges... I apply a thin layer of glue and spread it evenly into the wood grain of the root edge with a fingertip, and wipe off any excess, leaving a small rounded bead on the fin root. Takes maybe 30 seconds per fin. Same thing with the tube-- a thin, slightly rounded bead, spread thinly and evenly with a fingertip, and wipe off the excess on a paper towel... The tube can sit in the "trough" between the egg cups on the bottom of the egg carton with the glue applied end overhanging the end of the carton to dry, or you can stand the tube upright (glue end up) to tack up...

Once the glue is good and very tacky, apply a second THIN layer of yellow glue, thinly and evenly to the root edge of the fin... spread it thinly with your fingertip, and then carefully align the fin with the tube in the proper location, (both vertically along the tube length, and make sure it's aligned with the long axis of the tube, or correctly canted for rockets that are intended to spin on the way up). I usually put the rear tip of the trailing edge against the tube, make sure it's perpendicular to the tube surface (90 degrees straight out from the tube) and then gently rock the fin forward down onto the tube along the entire root edge length. Press it firmly against the tube and hold it very steady for about 30 seconds... the fin bonds to the tube almost instantly, so if you notice anything awry with the position or alignment, fix it IMMEDIATELY. Hold the fin still for about 30 seconds, and you'll find that the fin is nicely glued to the tube. I usually apply a VERY THIN layer of wood glue to any droplets of wood glue squeezed out from under the fin, and spread it with a fingertip along the root edge to make "mini-fillets" and spread the glue droplets smoothly and evenly along the surface of the tube and fin on both sides. Works great.

As soon as one fin is on, you're ready to move onto the next one... you can do all the fins at once-- on four fin rockets, I usually do them in pairs, and attach an angle-shaped piece of balsa or metal across the fins and clip it on with either clothespins or small clamps, to keep the fins perfectly aligned with one another. Then I do the remaining pair of fins. On three fin rockets, I usually just eyeball it and they come out pretty well aligned.

Set the rocket aside to dry-- I like to stand it up fins up to dry. The same technique works with white glue as well.

It's counterintuitive, but the thinner the glue layer, the stronger the bond, actually... excess glue contributes nothing to strength. So long as it's evenly applied, the joint will be strong. The double-glue joint method allows more glue to soak into the wood pores of the root edge of the fin for excellent strength. Gluing the fin directly to the tube in one pass squeezes most of the glue out from under the fin to either side of it (the fillet areas) and thus prevents much from soaking into the wood grain. How strong is a double-glue joint?? I can say from experience a properly done double-glue joint is stronger than either the paper-strengthened balsa and the paper body tube-- I had a rocket come down one time horizontally and impact sideways under power at high speed on very hard packed clay ground, and the front outer quarter of the fin from the tip inward along the leading edge disintegrated from the force of the impact, despite being papered... the fin finally was stuck in the ground enough that the fin broke loose from the rocket-- it didn't pop off the tube, though, it took the entire outer layer of glassine and underlying paper half the tube's thickness with it... IOW, the glue joint is STRONGER than the material being bonded-- no sense trying to go stronger than that, because either the balsa fin or the tube will fail before the glue joint does! BTW, the rocket was repaired by making a new fin, and building up the damaged area of the tube with another layer of paper glued in its place...

BTW, superglue is handy-- for hardening balsa transitions or nosecones, or even fins (but papering them works better IMHO). It's good for bonding detail parts or small fiddly bits on sci-fi looking rockets or scale rockets... but don't use it to attach fins and structurally-loaded components (like launch lugs, fins, etc.) Cyanoacrylate (super glue or CA) is weak in the shear plane, which is the forces of the rocket landing when the fins hit the ground. CA seals off the pores of balsa so wood glue applications cannot penetrate the wood and bond to it. CA also embrittles over time, leading to an ever weakening joint over time.

There's a number of threads on this topic that I've posted pictures of the process on, and also in many of my build threads-- just do a search for "Dr. Zooch Beta Builds", especially the EFT-1 and Vanguard Eagle threads... the pics and text are in there...

Later and hope this helps! OL JR

About thirty minutes after I posted this I downloaded "The Bible" and read all about it! As always all the input is greatly appreciated.

I don't know about how Gee, Hairy, did it. When I was a kid, the yellow stuff had just come out. We "discovered" our own way of gluing with it. We applied a thin amount of glue to the fin root and, using a finger tip, gently worked the glue into the wood grain. The glue went up the edges of the fin, too, about 1/8" or so. The fin was set aside to dry while the next was being done. I don't remember if we ever waited until the glue was totally dry or not before gluing it onto the rocket body. By forcing it into the grain, you get a *much* tougher joint than just letting it sit on the surface.

A friend built a D-Region Tomahawk (don't remember if this was a kit...). The first time he flew it, was when the "Econojet" engines had just come out. It was very exciting to have something with that much power! He built the first flight using thin ply fins, attaching them with epoxy (everybody knows this is the strongest way!). The fins stayed on for about 20' before peeling off and fluttering back down. We had never seen this before, an example of just how powerful those engines were! The next week he came back with the rocket rebuilt with balsa fins, glues on with SIG Titebond (yellow glue). It flew straight as an arrow, with intact fins.

Another tip about glue... I've got at least two, usually three bottles at one time. There's the fresh, thin stuff; great grain penetration, spreads easily, but is runny and won't stay in one place. Next is a used, older bottle. It's thickened somewhat with age. It's more like ketchup in thickness, pretty much stays where it's put, but will ooze. Good for putting fins on because it is very sticky and dries fairly fast. Last is the old bottle. It's thick like pudding (Oooooo...Pudding!). It is grumpy. It has to be dug out of the bottle and stays where it is put. Very good for making fillets and such. Don't ever use this stuff for gluing a motor mount, it will seize up instantly.

sooner.boomer said:

I don't know about how Gee, Hairy, did it. When I was a kid, the yellow stuff had just come out. We "discovered" our own way of gluing with it. We applied a thin amount of glue to the fin root and, using a finger tip, gently worked the glue into the wood grain. The glue went up the edges of the fin, too, about 1/8" or so. The fin was set aside to dry while the next was being done. I don't remember if we ever waited until the glue was totally dry or not before gluing it onto the rocket body. By forcing it into the grain, you get a *much* tougher joint than just letting it sit on the surface.

A friend built a D-Region Tomahawk (don't remember if this was a kit...). The first time he flew it, was when the "Econojet" engines had just come out. It was very exciting to have something with that much power! He built the first flight using thin ply fins, attaching them with epoxy (everybody knows this is the strongest way!). The fins stayed on for about 20' before peeling off and fluttering back down. We had never seen this before, an example of just how powerful those engines were! The next week he came back with the rocket rebuilt with balsa fins, glues on with SIG Titebond (yellow glue). It flew straight as an arrow, with intact fins.

Another tip about glue... I've got at least two, usually three bottles at one time. There's the fresh, thin stuff; great grain penetration, spreads easily, but is runny and won't stay in one place. Next is a used, older bottle. It's thickened somewhat with age. It's more like ketchup in thickness, pretty much stays where it's put, but will ooze. Good for putting fins on because it is very sticky and dries fairly fast. Last is the old bottle. It's thick like pudding (Oooooo...Pudding!). It is grumpy. It has to be dug out of the bottle and stays where it is put. Very good for making fillets and such. Don't ever use this stuff for gluing a motor mount, it will seize up instantly.

Click to expand...

Good points...

One other neat thing that bears mentioning is that for fillets, the thickened versions of glue work best... if you don't have a crusty old bottle like mentioned here, you can always pick up a bottle of the Titebond Moulding and Trim Wood Glue (TMTG) at your local mom-n-pop or other superior lumber or woodworking supply... (the big box stores that just imitate each other quit carrying it, since they just want to carry an extra row of regular wood glue-- simplifies their ordering/stocking system and they just stick more of what they already have on the shelf in its place-- and if you need it, you can go elsewhere or do without for all they care I guess).

The TMTG works really great for this... it's one step and done fillets... simply lay a bead down alongside the fin/tube joint on either side, and spread it with a slightly damp finger-- no muss, no fuss, goes on smooth as silk, what you see is what you get... no bubbles, voids, pinpricks, or shrinkage, no need to sand or do anything else to get perfect fillets, and you can do them ALL at the same time and set the rocket aside to dry-- it won't run, sag, or drip like regular wood glue or white glue. No fumes, wasted material, mixing, or nitrile gloves, tape, and alcohol to get smooth fillets like what's needed for epoxy fillets...

The stuff is really the cat's pajamas...

Later! OL JR
Later! OL JR