Outdoor capable laminations

[Brian Holcombe]

Thanks Sam, that confirms my gut feeling. I should have mentioned this from the get-go; I have a vacuum press and so Unibond 800 is an option in addition to other glues that require a vacuum press.

[William Fretwell]

I made a large complicated arch to hold a big solar panel off the back of a boat. I used 4 mahogany slices 3/16 thick glued up on forms with TB3 glue. Considerable clamping force was used on the forms and I expected some spring back but there was none! The initial stress in the laminations must have been very high but wood learns its new shape.
I examined the offcuts very recently from 3 years ago. There is no sign of creep even at a steep curve where it was cut. The curves are very strong. It is a fabulous technique to play with. You can incorporate hardware into the laminations for great strength.

I used TB3 as an economical choice as the total glue up area was considerable, I would not hesitate to use it again.

The whole arch was wrapped in carbon fibre cloth soaked in West epoxy, a messy dirty and most difficult procedure. This was given 3 coats of good marine varnish. It is very strong, flexible, and looks somewhat cool.

[Brian Holcombe]

Thanks William. This thread has been insightful. It's good to know that many processes can lead to a quality result. I'll study up some more before committing but I'm glad to hear of so many successes.

[Michael Feldman]

Sorry for being so late to the conversation, but I did an outdoor table with bentwood laminated legs and used a product called Ultra-CAT PPR Veneer Glue from VeneerSupplies.com. I had excellent results and the table has been outdoors since 2013 in coastal Rhode Island and it still looks great. I used black locust for the wood -- which I found a misery to work with -- but that's another story.

[andy bessette]

For bent wood lamination I would use WEST epoxy as I do on everything I build for yachts, priming first with neat resin/hardener mix, then applying a mix thickened with colloidal silica and microfibers, for the actual glue (to insure 100% void-free contact). If the assemblies are large, complex or time consuming to coat, or if ambient temperatures are high, I would use slow or extra slow hardener, to provide sufficient pot life and assembly time.

Care must be taken to avoid contaminating adjacent surfaces and to cleanup prior to cure, as the cured epoxy is very difficult to remove, and can stain the surface slightly. I clean dry, first, then with acetone. After curing, scraping or sanding can remove excess. On oily wood, like teak, I pre-wash the glue surfaces using acetone. Never thin epoxy or incorporate any additives not specifically recommended by the formulator.

[Brian Holcombe]

Thanks gents, this is an old post now. I'll probably check in on that chair sometime this spring to see how it's doing and attend to any issues that have cropped up if any.

[Stanley Covington]

Phenol-resorcinal resin is still the standard for exterior structural glulams. The Europeans have approved some other types, but aliphatic resins are not approved for exterior structural use.

PR is very good, easily applied, not especially dangerous or toxic to use, great strength and bond, absolutely waterproof, no measurable creep in the adhesive itself, but it is a dark brown/purple color.

Epoxy is just as good performance-wise, and looks a lot better, but is more time-intensive to mix, sensitive to mixing ratio, not as easy to apply as PR, doesn't play as nicely with gang clamps, and is decidedly more expensive. Probably not an issue for the volume Brian is considering.

Therefore, PR is the standard laminating adhesive for commercially-produced exterior-grade structural glulams.

I would use epoxy.

[andy bessette]

...PR is very good, easily applied, not especially dangerous or toxic to use, great strength and bond, absolutely waterproof, no measurable creep in the adhesive itself, but it is a dark brown/purple color...

...and requires nearly perfectly mating surfaces, as it has no gap-filling capability.

[Stanley Covington]

...and requires nearly perfectly mating surfaces, as it has no gap-filling capability.

"Perfectly mating" is an exaggeration. PR works fine for milled lumber.

PR does have some gap-filling capability, not as much as epoxy, but it is certainly adequate in many countries millions of times a day for milled lumber.

I doubt Brian will need to span 1/4" gaps.

[John C Cox]

I can completely understand the desire to stay away from hot bending mahogany... Of the woods I have bent - it is easily the least predictable and can be massively cantankerous with no good reason or prediction that it will be such.

Oak on the other hand is the opposite in that particular regard... Ridiculously easy to hot bend for such otherwise cranky wood...

So... If you get into this and do decide you like the idea of bent wood over glue for outdoor use - plain straight grained white oak would be my choice.

If not - I agree with the others on seeking out a good Sailboat structural epoxy... West Systems is great..

A caution.... The stuff soaks to china... And it can run up pores and end grain like mad, soak right up into the wood and diasappear... The typical procedure is to soak the glue joint grain ahead of gluing - to make sure it doesn't all soak into the wood and leave a dry/starved joint. The other down side is that because it can soak so deep - it typically darkens woods considerably.. And sometimes even quite a distance from the glue joint... It can also leave dark glue lines like Superglue often does... Especially if you are gluing end grain. I wrecked a guitar top when epoxy soaked into end grain through the rosette channel and wicked 5" up the grain... Sometimes it's OK - sometimes it's not. Test on scrap to make sure....

[andy bessette]

"...requires nearly perfectly mating surfaces..."

"Perfectly mating" is an exaggeration...
I doubt Brian will need to span 1/4" gaps.

You misquoted me...and exaggerated.

[Stanley Covington]

"...requires nearly perfectly mating surfaces..."



You misquoted me...and exaggerated.

Please observe that your post was quoted in its entirety. No misquote.

You did not mention distances, so exaggeration did not occur.

PR can handle 1/8" gaps of limited length, but not 1/4". I believe that epoxy can handle much larger gaps if it is contained in place until it hardens. Is that not the case?