Wall Construction Question

[Capebuild]

Hello. I'm getting ready to "laminate" the 3 sections that will make up the 2 sidewalls. I'm using .238 ply for the outside, .750 MDO ply for the middle skeleton sections and .157 ply for the interior sections. I've got 3 coats of urethane on all surfaces of these ply sheets ...using the mix 25%/75% for first layer, 50/50 for second and 100% urethane for 3rd coat.

My question is would I be better off using TiteBond glue or PL3XPremium given that the surfaces are all coated with urethane?
I'm also planning on using 1/2 crown staples to adhere the outer .237 layer to the MDO and just using "weights" to adhere the interior wall layer to the MDO (not wanting to put any nail holes or staple holes in the visible interior wall sections).

Just looking for opinions on this.......or suggestions in general.....before I get down to it.

Thanks!

John

[eLink]

I think normally Titebond would be used for large flat, even surfaces like this. But wood glue of any kind requires the mating surfaces to be clean bare wood.
Just thinking out loud here: My first thought would be to sand everything down again, but then you would likely end up with uneven mating surfaces. Maybe an epoxy glue alternative?
I'm curious to hear what others here think.

[halfdome, Danny]

John, as a life long Cabinetmaker I've never glued finished wood to finished wood together since the wood glues bond bare wood fibers to bare wood fibers and become stronger than wood itself.
It appears your attempting to glue 3 coats of finish to finish.
I can't disagree with the first reply, just wish I had an easy answer to your situation.
Maybe get some scrap of your finished materials and do some glue up tests subjecting them to the conditions that your teardrop will experience.
Danny

[Capebuild]

Thanks elink and Danny for the followup suggestions. I didn't think it would be a good idea to use titebond since the surfaces had already been coated with urethane. So you have confirmed my thoughts. I'll try your suggestion, Danny, and do a test with the PL Premium on some coated surfaces... I have a feeling that would hold up, but we'll see and I'll report back.

Thanks again

John

[GarthB]

Titebond has a polyurethane glue that works well for finished surfaces that I've used on small finished items. I don't believe they sell it in large containers though. The biggest drawback I've found is that its pretty messy and foams a lot as it cures.

http://www.titebond.com/product/glues/78b95323-a09e-45c2-b5ad-7ea90b13c6ca

I used PL Premium to add a 1/2" layer to my existing 3/4" floor that already had been polyurethaned, however, I used a lot of screws as a secondary measure in order to ensure that it was all laying evenly since PL is fairly thick and doesn't spread as easily as wood glue. I suspect you won't want to add many (if any) screws to your wall construction.

Sanding and using wood glue will probably give you the best strength and minimize voids though.

[TimC]

John... maybe if you can spare the wall height you could use a router trim bit and take a sixteenth off the edge of the wall and with a jig do the same on the floor. That would result in an 1/8 inch shorter wall. That way you should have fresh wood to absorb the glue. I'm not sure how that would effect your tab and slot construction method.

Some woodworkers, like Izzy Swan, will cut the glue on ply edges with water (one to one) to help it penetrate then before that sets they use full strength glue to bond those surfaces.

[QueticoBill]

I think short of removing the finish on the surfaces to be glued, you are in an unwinnable situation. My condolences.

Actually, 3M makes a VHB double sided tape - very high bond - that only works on sealed surfaces. You might get a "gap" because it has some thickness, but I think it might work. Call 3M, they have a great tech support staff who answer the phone and talk!

[noseoil]

PL adhesive would be the best choice in this case. The titebond won't stick to urethane (urethane would act as a mold-release agent as the titebond is water based), but the PL should do ok if the joint has some pins to lock things until it can set up. Use some clamps if possible & let the joint cure completely before moving anything.

I would scuff-sand the joints to give a rough surface for the PL to key into (the scratches) & give a bit more bonding surface. Hopefully, the PL will adhere well enough to form a good bond, but a test sample would be the only way to see if it's creating a bond strong enough for your needs.

Next time, leave the glue surfaces bare for titebond, so it can soak into the wood & do it's job.

[tony.latham]

Damn good question.

I have used PL3 to glue a skin on a (bare) sandwich wall and won't do it again. I used a notched trowel to get good distribution. The problem is that even with lots of weight and clamps, the skin wants to float on the adhesive. It's just too thick. You can't give it enough squeeze.

I think I would use epoxy but would scuff the urethane first with a random orbital. I might apply it to both surfaces too...



Tony

[Capebuild]

Thanks all, again, for the follow up. I keep asking myself why I went ahead and urethaned these surfaces prior to installation... but one of those things, hindsight being 20/20. A lesson learned.

That being said, needless to say I've been pondering this since yesterday. Had a conversation with a Loctite rep this morning. He suggested I look into Loctite adhesive Teroson MS939NA. Supposedly has good bonding capabilities. I just printed out the data sheet...so still researching it. I also asked him how it compared with PL3X Premium. He said they're similar but different animals. So will print out that data sheet and compare. And I'm also doing some testing with scraps. But one thing I've read is to make sure the urethane is really dried (cured). 48 hours was mentioned, but being so humid I think I'll give it longer before bonding the 3 wall layers together. I also think I'll give the urethaned surfaces a light sanding with the orbital sander. If I think of it like using super glue, you have better results if the mating surfaces are smooth. The other thing about doing the orbital sander thing is it kind of gets rid of that initial urethane film on the surface. That is where the failure (delimitation) would occur, acting like a wax paper release; just pulling away from the sealed surface. Also.... one other thing... I'm thinking of drilling a series of small (1/4") holes in mating surfaces on slightly different angles.... so when the adhesive is applied it would fill those holes and act like little anchors for the adhesive. That might be overkill, so still thinking about it. The outer ply will get crown stapled to the middle ply so not too worried about that bond. It's the interior ply that gets bonded to the middle skeletonized sheet. That will not have any staples or other mechanical fastening method, as I don't want to see any fill holes.

Anyway... a real learning experience (aren't they all)..... will report back with results.

John

[western traveler]

John,
I keep running your question through my mind. I actually stopped working on my build to come in and comment on this.

If I am understanding your question correctly you are not talking attaching ends of panel joints to ceiling or floor etc, that would be easy, just strip the to bare wood.
Are you asking about laminating the 3 sheets together to form your wall panels.

If this is correct Quantico Bill said it. There is nothing short of stripping the coatings off the surface areas to be glued that I can think of to fix this. Everything else is a shortcut to move on with your build.

I come from the marine industry where we dealt continually with laminated surfaces. We sometimes took shortcuts because we had to get off the dock. We took them knowing we would be back addressing the issue again. Sometimes we got a few years out of it and sometimes not.

What effected it was weather, heat and cold, humidity or lack of, along with vibration and delamination followed.


A very tough spot to be in.

[western traveler]

Sorry John,
I am a slow slow typist. I was composing while you were posting.

[QueticoBill]

Still think you ought to try the VHB tape but remembered Pliobond:
https://www.ellsworth.com/products/adhe ... .5-pt-can/

[saltydawg]

I used the 939 to bond the panels of my trailer together, its incredible stuff. Like 500 % elongation before it fails.

It is also not very expensive when you think the 22 oz tube is like 2 or 3 normal tubes of caulk.

Here is a vid of styromax testing it.
https://www.youtube.com/watch?v=FBgdV8YT1Es&t=120s

[MickinOz]

Have you got a couple scraps of polyurethane coated plywood?
Try SIKA Techgrip on it. Like the titebond glue it's urethane, so it'll foam if loose, so clamp it or staple it.
I expect you will be surprised.
I didn't intend to, but a couple times I found it necessary to glue poly'd wood together. I used all Sika Techgrip in the build.
Seemed just as strong as bare wood to bare wood.
Remember, a glued joint between two pieces of plywood is no stronger than the glue that holds the veneers together.