double glue joint questions

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neil_w

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I've always used double glue joints with my wood glue, building balsa and paper rockets. It occurred to me recently though that there's an aspect I don't understand. Most instructions say to apply the first layer of glue, and then wait... well, an unspecified length of time. Then apply the second layer and attach.

The question is how long to wait before applying the second layer. Is it necessary for that first layer to still be wet, or tacky, or something? If the first layer dries completely before the second layer is applied, does that help, hurt, or not affect the joint?

In my own practice, I realize that I'm not particularly consistent with how long I wait between the first and second glue layers. Does it matter?
 
I normally wait until the first coat is not really tacky anymore, but not quite dry yet, if that makes sense. Seems to work so far, but I'm not going crazy on thrust or anything either.
 
I normally get in a hurry and only wait about 30 minutes. This last one I did I let it dry overnight and it made a significant difference. Set was almost immediate.
(I am working with Gorilla Glue. Others may work differently)
 
If you read the instructions in the Handbook of Model Rocketry it tells us to wait until the first application is dry to the touch. then reactivate with the second glue application.

It's what I've done since the 1960's and seems to do an outstanding job:)
 
I normally get in a hurry and only wait about 30 minutes. This last one I did I let it dry overnight and it made a significant difference. Set was almost immediate.
(I am working with Gorilla Glue. Others may work differently)

If you read the instructions in the Handbook of Model Rocketry it tells us to wait until the first application is dry to the touch. then reactivate with the second glue application. It's what I've done since the 1960's and seems to do an outstanding job:)

This is where I sheepishly admit that I don't own the handbook.:facepalm: Still, I would have wondered about whether waiting longer than "dry to the touch" (like overnight) was good bad or otherwise. It sounds like letting it dry is OK.

Thanks all.
 
Here's a way to do practically all 3 or 4 fins pretty much without waiting a long time between. 1st and foremost, I cut the glassine away from the tube where the fins are going. All of those areas get an even spread of wood glue and I let it dry totally. Usually overnight but that's not etched in stone. Sand each area slightly till the boogers are gone. I'll give each fin a nice layer of glue and set them aside for 4-5 minutes (or until they just get started to look like they're starting to set up a bit. At this point, the glue is in such a state (depending on how large they are) that you can attach them without them falling off. Just apply a little pressure and make sure they are seated well. Start with the first one you put glue on and move to the next fin and keep going there. Checking alignment all the way through. This is where a set of those Qualman jigs come in handy, but necessarily a must.
 
Tnis is interesting. I never do any if the procedures, but should try them. I put a layer of glue on the root edge and wipe 90% away with my finger. I wait until it gets tacky then I set the fin on the desire location on the tube. After it sets overnight I come back and apply fillets.
 
Tnis is interesting. I never do any if the procedures, but should try them. I put a layer of glue on the root edge and wipe 90% away with my finger. I wait until it gets tacky then I set the fin on the desire location on the tube. After it sets overnight I come back and apply fillets.

I do fillets the next day also. Since I am never in a rush to build, I can allow plenty of drying time for any application.
 
Seems like in one set of Estes instructions, or somewhere...well I got it in my head anyway that I saw to apply a thin later to both surfaces and let dry or stand for a minute or two, then another thin layer and stick the fin to the body. I got to where I would count to sixty while doing other things, then do the second application and stick the fins. Grabs nicely and I was able to move on. I have only had one fin break off and that was me being rough with it getting it out of my storage bin. But most of my LPR have very few flights on them so YMMV.
 
Seems like in one set of Estes instructions, or somewhere...well I got it in my head anyway that I saw to apply a thin later to both surfaces and let dry or stand for a minute or two, then another thin layer and stick the fin to the body. I got to where I would count to sixty while doing other things, then do the second application and stick the fins.

Yeah, that's where I was getting confused. "A minute or two" is very different from "until dry". I'm still not sure I understand the adhesive theory of the double glue joint.
 
I'm still not sure I understand the adhesive theory of the double glue joint.

You take a fin with some fresh glue on it, stick it on an un-prepped tube, and see how long it takes to hold it in place while you wait for it to stick by itself....
case dismissed.
 
Yeah, that's where I was getting confused. "A minute or two" is very different from "until dry". I'm still not sure I understand the adhesive theory of the double glue joint.

I always leave enough on the root edge to just fill the grain (a similar amount on the tube) and wait at least until the surface of the glue no longer looks wet. This works well for me since I usually have a hard time even waiting that long. I have had some that ended up sitting over night or longer, I still had good results but I haven't noted whether one method stuck better than another.
 
You take a fin with some fresh glue on it, stick it on an un-prepped tube, and see how long it takes to hold it in place while you wait for it to stick by itself....
case dismissed.

What I meant was, why does it work? Does the wood glue stick better to the other wood glue? Does wet wood glue stick better to dry wood glue than directly to wood? Etc. I've always done it because that's what everyone says to do, and I do trust that it works, but I don't really get the theory behind it.
 
What I meant was, why does it work? Does the wood glue stick better to the other wood glue? Does wet wood glue stick better to dry wood glue than directly to wood? Etc. I've always done it because that's what everyone says to do, and I do trust that it works, but I don't really get the theory behind it.

All of the above. Including partially dried (tacky but kinda wet stage) to totally dry. Not sure if it sticks better, but your dried glue has set very well already.
 
I heard from an experienced old school wood worker that a properly done double glue joint can be up to 10 times stronger. Must let the first coat dry completely on each prepped surface. The all kinds of bonding science goes on using pure glue to glue joint.

Made me feel very inadequate with my rough up the glass line tube, two dots of superglue on each end, a line of Ileen's tacky glue in between and then four fins on lickety split!
 
You take a fin with some fresh glue on it, stick it on an un-prepped tube, and see how long it takes to hold it in place while you wait for it to stick by itself....
case dismissed.

Reminds me of instructions for microwaving a baked potato.
Get two identical potatoes.
Put one in microwave set on high for 1 hours.
Turn on and wait for potato to explode. Write down time T
Remove, clean oven
Put second potato in over on high. Set time for T minus 1
 
You take a fin with some fresh glue on it, stick it on an un-prepped tube, and see how long it takes to hold it in place while you wait for it to stick by itself....
case dismissed.

Reminds me of instructions for microwaving a baked potato.
Get two identical potatoes.
Put one in microwave set on high for 1 hours.
Turn on and wait for potato to explode. Write down time T
Remove, clean oven
Put second potato in over on high. Set time for T minus 1
 
Reminds me of instructions for microwaving a baked potato.
Get two identical potatoes.
Put one in microwave set on high for 1 hours.
Turn on and wait for potato to explode. Write down time T
Remove, clean oven
Put second potato in over on high. Set time for T minus 1

Sounds like the instructions to torque a head bolt; "tighten it 'till it strips, then back off a quarter turn".
 
What I meant was, why does it work? Does the wood glue stick better to the other wood glue? Does wet wood glue stick better to dry wood glue than directly to wood? Etc. I've always done it because that's what everyone says to do, and I do trust that it works, but I don't really get the theory behind it.

Wood glue, white glue, PVA, aliphatic resins are polymer glues. They stick by having long strings of molecules getting linked together and holding tightly together. By putting a thin layer on your parts first you are creating a patch of gluey strings on the fin and body tube that will hold tight to each surface. When you put the 2nd line of glue, you are giving the other side of the patch something to grab onto. Think of it like the glue is velcro. Instead of putting a hook part on the fin and the fuzzy part on the tube, you are putting fuzzy sides on each part. The 2nd glue application is like a double sided hooked strip. When the glue dries the polymers twist up real good and make a strong bond.

kj
 
Since faster-setting glues like Titebond and Titebond II will dry in 15 - 30 minutes can the next layer be applied after this amount of time, or does it have to wait till it's fully cured in 24 hours?
 
Since faster-setting glues like Titebond and Titebond II will dry in 15 - 30 minutes can the next layer be applied after this amount of time, or does it have to wait till it's fully cured in 24 hours?

They might dry to the touch, but they aren't completely dry for many hours later. Very thin layers can dry quicker like when you are treating the root of the fin or the tube. But when you are applying the fin, the "new glue" needs to still be somewhat moist / undry if you will. You can see when the glue is dry on the tube and that can be within a couple of hours (at best) then with fresh glue on the fin, I usually wait until it's just starting to set up a bit (still moist yet starting to dry) and apply to the tube. It'll usually stick within a few seconds and you can let go of it to apply the next fin. Your average 4FNC rocket can be finned within a couple of minutes. But be cautious because they will still pull off or get canted. I usually keep an eye on them for the next 5-10 minutes. This is possible when thew rocket is upside down sitting still on the bench.
 
They might dry to the touch, but they aren't completely dry for many hours later. Very thin layers can dry quicker like when you are treating the root of the fin or the tube. But when you are applying the fin, the "new glue" needs to still be somewhat moist / undry if you will. You can see when the glue is dry on the tube and that can be within a couple of hours (at best) then with fresh glue on the fin, I usually wait until it's just starting to set up a bit (still moist yet starting to dry) and apply to the tube. It'll usually stick within a few seconds and you can let go of it to apply the next fin. Your average 4FNC rocket can be finned within a couple of minutes. But be cautious because they will still pull off or get canted. I usually keep an eye on them for the next 5-10 minutes. This is possible when thew rocket is upside down sitting still on the bench.

Patience was never one of my virtues, but since I want to do it right I'll wait for Titebond's 24 hour full-cure time.
Sheesh, it just seems like I spend most of my build time just waiting for glue to dry even when working on several models at once.
 
Patience was never one of my virtues, but since I want to do it right I'll wait for Titebond's 24 hour full-cure time.
Sheesh, it just seems like I spend most of my build time just waiting for glue to dry even when working on several models at once.

We all share that burden. But nothing done quickly is ever done well. You will develop patience sooner or later or continue to disappoint yourself. It a good part of the reason why we DO multiple builds. I learned to get into the habit of walking away when glue is drying and apply my time to something else needed doing. Start with gluing stuff or sealer treatments, putty fills and what not, then start turning a nose cone or two or policing a badly abuse area of the work shop. If there's nothing else to do in there, walk away and go wash those dirty dishes.
 
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