Double-glue joint

After following this thread:
https://www.rocketryforum.com/showthread.php?s=&threadid=139

I came to wonder about something I read in the Handbook of Model Rocketry. The subject of double-glue joints and if it works as good as this book says.

Just to clarify, "...when gluing porous materials such as paper or wood you coat both surfaces with a layer of glue or bonding agent and let both surfaces dry. Then coat both surfaces again and join them together. The first coat of glue on both surfaces penetrates the pores of the material. The second glue coat is then free to join with the first coat and with the second coat on the other surface. A double glue joint will be so strong that the materials will break or tear before the glue joint turns loose."

"When gluing two pieces of plastic or other nonporous material, use a variation of the double-glue joint. Coat both surfaces with the bonding agent. Since the surfaces don't have pores, you don't need to apply two separate coats of glue to the surfaces. But the cement will soften each surface or adhere to each surface so that the bonding agent will then bond to itself when you bring the two pieces together."

Unless I misunderstood something in the other thread, the posts in it seem to contradict this double-glue joint method.

Most prolly because wood glue on wood glue may soften up the old layer and do the described type of bonding. Then again I don't see the advantage in contrast to just putting wood glue on one part and pressing it against the other. It will still soak in and what not and create the desired strong bond all by itself.

But I can tell you that cured epoxy is just like a normal plastic which nothing sticks to very well. That's why you don't want to but a layer of epoxy between your lamination and the fin.

Oliver

Oliver Arend said:

Most prolly because wood glue on wood glue may soften up the old layer and do the described type of bonding. Then again I don't see the advantage in contrast to just putting wood glue on one part and pressing it against the other. It will still soak in and what not and create the desired strong bond all by itself.

But I can tell you that cured epoxy is just like a normal plastic which nothing sticks to very well. That's why you don't want to but a layer of epoxy between your lamination and the fin.

Oliver

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Good name. My son has the same. I don't think I would use epoxy unless I applied a fiberglass layer over a model for a contact cure. Even so I would still have the urge to glass the fins for the contact bond.
I wish I still had my old job to pull vacuum on parts and oven cure them. I would even be able to utilize different core material for fins.
Oh well.

Milo said:

After following this thread:
https://www.rocketryforum.com/showthread.php?s=&threadid=139

I came to wonder (snip)

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I saw nothing in that thread suggesting any type of gluing method...wrong thread? Anyway, that's a slight bit overkill on the glue in my opinion. As a rule, I'll mark the tube where the fin attaches and remove the glassine (low power models) and add glue to the raw material. Let that dry thoroughly, then apple a bead of glue on the fin root. I find it best to wait a little bit before attaching it to the tube. Say about, the time it just gets started setting up but still sticky/tacky. If you time this just right, you can get all fins on at once. Then there is the glue of choice here to consider. When that book was written, white glue was prolly the go-to glue at that time. Since we now have better choices like Titebond 2 and such, all that extra gluing really isn't necessary. I'm all about LPR rocketry, so I've been down this road a really long time. My friend goes as far as to poke holes in the tube with a pin so the glue forms a rivet of sorts inside the tube. To each his own I guess, but in all the years I have done this, the way I described, I have never had fin shear from the tube.

Like Gary I've been building model rockets for a very long time:
The reason double glue joints work as discribed in the Handbook of Model Rocketry is the second application (only needed on one joining part (fin root or body tube) reactivates the dryed first glue application by forming a Chemical bond, not just a Mechanical bond that is formed on non-porous surfaces.
Let me again stress: The completed double glue Joint applied to any wood to wood or cardboard (processed wood) is Stronger then either of the materials being joined. In simplier terms: the Fin will break or the body tube will tear before the glue joint fails.
White or Yellow double glue joints ARE the Strongest available Wood to Wood joining material period.l

I have adopted the double-glue joint on all my LPR builds too, and have taught it to my kids. One other benefit I've noticed from this, and I think Gary alluded to it, is the faster set-up time. Unlike him, however, I don't wait for it to get tacky on the second app, but maybe I'll give that a try. Even without the wait, however, the part tacks almost immediately with none of that slipperiness that can happen on the first application. I was skeptical of it too, and think I even posted something here about it many years ago- to me it seemed counterintuitive that you're filling in all those valuable pores with something non-porous...but at the molecular level is where the magic happens.

Gary,

How do you go about removing the glassine from the body tube where the fin goes?

Best regards,
Brian

BRS Hobbies said:

Gary,

How do you go about removing the glassine from the body tube where the fin goes?
Best regards,
Brian

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Brian, what works for me is to use the line you drew on the tube for fin placement, and take a very sharp (new preferably) knife and carefully score the glassine down to the tube surface. Some people will cut ON that line and make the 2nd cut to the right (or left) of it, keeping with that one side all the way around the tube. I cut on both sides of the line and remove only enough glassine that equals the fin thickness. One important thing to remember is, it's better to cut a little shorter than the length of the root edge. Cut too far, and you'll be patching the tube. The pic shows a little bit of what a treated glue joint will look like. Look at the brown tube at the top.

soopirV said:

Gary alluded to it, is the faster set-up time. Unlike him, however, I don't wait for it to get tacky on the second app, but maybe I'll give that a try. Even without the wait, however, the part tacks almost immediately with none of that slipperiness that can happen on the first application.

Click to expand...

I suppose I should clarify here just a bit. In my neck of the woods, we deal with a lot of humidity. That being said, wood glues take longer to dry. On a good dry day, I too, can tack on a fin instantly. But when it's humid, I find I have to wait a bit, so the bead of glue on the fin root starts to set up a bit to achieve that quick hold. Just something I had to learn by trial and error.