Polyurethane Glue and Domino Joinery

[jeffinsgf]

Opinions? I use polyurethane (Gorilla) glue pretty frequently. I recently used it on a project with a lot of Domino joints and started over-thinking the whole situation. The first advantage I see is poly's expansion. Even die-hard Domino fans have to admit, they aren't exactly the tidiest glue joint fit. They're not even close to the fit most serious hand-tool woodworkers expect on a mortise and tenon. PVA glues rely heavily on wood to wood contact...poly...not so much.

Next advantage is application. I used an acid brush to spread the glue in the mortise and wrapped the Dominos in a wet paper towel a few minutes before assembly. When I use PVA, I apply to both the mortise and the Domino, which takes longer and increases the chance of getting too much in the joint.

Finally, clean up. I am firmly in the camp of "leave it alone until it's dried and then clean it off with a chisel" (regardless which kind of glue). I find dried poly comes off easier and cleaner than dried PVA. There's less final residue and it requires less sanding to ensure no ghosts in the finish.

So, I'm leaning toward using polyurethane on all Domino joinery in the future. I'd love to hear from anyone with an opinion on the matter, one way or the other.

 

[Frank-Jan]

I think this is the first thread I have read complaining the domino mortices are not tight enough.

 

[Michael Kellough]

I usually only glue Dominoes when one of the parts presents end grain.

I use PU glue even less often. The wet glue is a mess to get off my hands.
And it seems to be much weaker than aliphatic resin glues.

Isn’t it difficult to put a swollen Domino (“wrapped in a wet paper towel”) into the mortise?

I agree that dry PU glue is easier to remove.

To deal with glue squeeze out I try to turn clamped assemblies so that the glue line is vertical. Any wet glue runs along the joint instead of across the face of the board. There are several ways to deal with the  excess glue after that.

 

[Michael Kellough]

I think this is the first thread I have read complaining the domino mortices are not tight enough.

I’ve put Dominoes in the microwave to shrink them but I’ve never needed to make them swell, even when not using glue.

If Jeff’s mortises are noticeably bigger than the Dominoes maybe his bit or machine are bad?

I think he just means the edges of the Dominoes which not match the round profile made by the bit. Maybe deliberately in order to avoid hydraulic blow out. If a custom Domino perfectly matched the mortise you’d have to be sure the glue applied was just enough.

 

[MikeGE]

So, I'm leaning toward using polyurethane on all Domino joinery in the future. I'd love to hear from anyone with an opinion on the matter, one way or the other.

Here are two YouTube videos from Bradshaw Joinery showing his use of PU glue on projects. 
 

 

[Packard]

I was an early adopter of the PU hot glue gun system.  It was expensive to get into it because the glue gun cost a couple of hundred dollars, and there was minimum purchase quantity to buy the glue sticks.  But the reviews (in industrial magazines) were terrific, with very high strength and a super nifty glue gun that was not wired to the charging holder. 

I invested in this because I thought it would speed up production in my picture frame shop.  It read like it would.  I could glue and join a frame and then start fitting the art/mat/glass and mounting boards immediately. 

We were using pre-finished moldings,  The PU glue left a black line at the joint and so was unusable.  I think I’ve used it twice after that.  I’m not sure where it is, but in my basement somewhere.

My point is, when entering a new tech, the information available is what applies to the type of work the reviewer is performing.  It might not mention some crucial aspect of the system that could mean thumbs down on the product for your application.

What is the open time for this adhesive?  I tried making a furniture repair (damaged by the moving company) using Gorilla glue (polyurethane).  The glue expands as it cures and pushed the joint open.  A strong joint, so it is impossible to take apart and re-glue.  An 1/8” gap where there should not be any gap.

If I knew about the expanding, I would have used a beefy clamp.  It was a dowel joint that appeared to be solid.  I don’t think I used a clamp—I think I used a bungee cord wrapped around the piece. 

Make some tests before committing to the new glue. 

What advantages does it hold for you?  I use Woodworkers’ III for general glue-ups.  I like the long open time.  I use hide glue for cane work, and Corner Weld (a picture framers’ glue) which has a extremely short open time and tacks up in a minute or less.  Handy when gluing up picture frames where assembly is almost instantaneous.

 

[jeffinsgf]

I think this is the first thread I have read complaining the domino mortices are not tight enough.

I’ve put Dominoes in the microwave to shrink them but I’ve never needed to make them swell, even when not using glue.

If Jeff’s mortises are noticeably bigger than the Dominoes maybe his bit or machine are bad?

I think he just means the edges of the Dominoes which not match the round profile made by the bit. Maybe deliberately in order to avoid hydraulic blow out. If a custom Domino perfectly matched the mortise you’d have to be sure the glue applied was just enough.

My Domino is fine. My fit on the thickness is spot on. Like everyone else, I have to use pliers to extract dry fit Dominos. The issue is the ends. I understand the air gaps for hydraulic expansion, but that detracts from mechanical strength of the joint. And if you switch to a wider setting, you're getting no mechanical strength in that direction at all. I have found the expansion of polyurethane to fill those gaps and create what I think are better bonds.

I don't "need" to make the Dominos swell. The directions for Gorilla (and other polyurethanes) say to dampen one surface and apply glue to the other. Wrapping the required Dominos in a wet paper towel was my quick and dirty way to dampen the surface while I was applying the glue to the mortises.

 

[jeffinsgf]

I think this is the first thread I have read complaining the domino mortices are not tight enough.

Not complaining at all. Domino mortises are perfectly fine in thickness, but by design they do not fit very well on the ends. All I'm saying is polyurethane glue can help create a better joint by filling the designed gaps as it cures.

 

[Packard]

You are suggesting that filling the slot at the ends of the domino will add strength?

I don’t see how it would.  And of that gap were filled with glue, the PU would try to force the domino out of the slot making new demands on clamping.

Has anyone ever done testing on dominoes, dowels or tenons suggesting that PU offers greater strength than other glues?

If that gap causes weakness, then I should drill shallower holes for my dowels.  I always leave about 1/8th of an inch gap at both ends of the dowels.

 

[squall_line]

You are suggesting that filling the slot at the ends of the domino will add strength?

I don’t see how it would.  And of that gap were filled with glue, the PU would try to force the domino out of the slot making new demands on clamping.

Has anyone ever done testing on dominoes, dowels or tenons suggesting that PU offers greater strength than other glues?

If that gap causes weakness, then I should drill shallower holes for my dowels.  I always leave about 1/8th of an inch gap at both ends of the dowels.

A wide mortise could potentially weaken shear strength by providing an air gap for the tenon to move laterally, but I would think that it would depend on the grain orientation of the joint as well as the loads encountered.

I thought that at least one method of mounting breadboard ends to account for expansion/contraction was to mill wider mortises on the table side and leave out the glue on most of the wide mortises while leaving the breadboard end tight and glued, but I've never actually built a table, I've only read about it.

 

[ChuckM]

This fellow uses polyurethane glue for furniture making though I'm not aware of people I know who use it:
https://www.finewoodworking.com/2016/11/02/polyurethane-is-my-go-to-glue

I once was given a bottle of such glue at a trade show, probably 15 years ago. I never got to using it before I threw it away, so I couldn't say how good it was.

I have had no concerns using yellow glue in any Domino joinery. As part of my routine, all joinery done regardless of its type is tight, cut whether by hand or by machine, so I guess I don't come to wondering if I need gap-filling glue or not in my work. For joinery work, I only use two types of glue: PVA or Old Brown Glue.

Norm Abram did use polyurethane glue in a couple of outdoor projects.

 

[jeffinsgf]

You are suggesting that filling the slot at the ends of the domino will add strength?

I don’t see how it would.  And of that gap were filled with glue, the PU would try to force the domino out of the slot making new demands on clamping.

Has anyone ever done testing on dominoes, dowels or tenons suggesting that PU offers greater strength than other glues?

If that gap causes weakness, then I should drill shallower holes for my dowels.  I always leave about 1/8th of an inch gap at both ends of the dowels.

I suppose I should have said edges rather than ends. I am talking about the rounded edges of the mortise and the rounded edges of the Dominos not fitting tightly to each other. I think, with no empirical evidence, that polyurethane glue makes a better joint than PVA, and even if it is only as good... it is faster to spread, clamp and clean up.

I will agree that cleaning it off your hands is impossible. That's why I use gloves.

 

[mrFinpgh]

I have used it to good effect for veneering (it's good for repairing bubbles) but generally I prefer not to use it.  It's messy, and the chemicals from the curing process are a bit nasty to spend time around.

I don't think it adds anything structurally to the M+T joint if you are counting on the expanding foam to provide support/strength.  Generally a little room on either side of the joint is fine as the long grain to long grain connection is where all the holding power is coming from.  I guess if all other systems failed, the tenon to mortise connection might be providing something, but if I need that kind of stability, I'm probably not using Domino joinery in the first place and I'm probably drawboring that joint.

I much prefer to clean excess glue off ASAP.  Usually scooping off the bulk and hitting the rest with a toothbrush and some water, wiping it dry once I'm done. This doesn't hurt the joint, and I've never had any issues with finishing as long as I really get it all cleaned up.

 

[luvmytoolz]

I've used the Gorilla PU on some projects, and apart from the nuisance that it dries in the bottle if not used up quick enough over time, found it gave no filling strength at all which was a surprise. Once you break the touch outer skin, it's very soft inside and takes hardly any force to compress.

For wood to wood gluing it's great, apart from the glue line on some timbers, but doesn't add anything apart from being good for moist/wet areas.

 

[Crazyraceguy]

I have never really seen the need for Poly glue, like Gorilla, for what I do. The only time I ever did was when gluing steel threaded rods into some wooden spindles. They were a turning project that held an oval mirror between them.
I don't do outdoor projects and Tite-bond has always had a product that suited my needs. Usually 2 or 3  or the thick stuff for trim.

 

[luvmytoolz]

Yeah TB's my go to glue, I get invisible seams, and the joint is wickedly strong. It'd be nice if they released a TB1 with a long open time though as I do a lot of bundles of laminated strips, and it gets very messy and tricky very quickly.

A mate was asking what glues to use for some projects so I gave him cutoffs from some glue ups and told him to break them. All of them broke in the wood, none at the joint. He was impressed to say the least!

 

[TinyShop]

Yeah TB's my go to glue, I get invisible seams, and the joint is wickedly strong. It'd be nice if they released a TB1 with a long open time though as I do a lot of bundles of laminated strips, and it gets very messy and tricky very quickly.

A mate was asking what glues to use for some projects so I gave him cutoffs from some glue ups and told him to break them. All of them broke in the wood, none at the joint. He was impressed to say the least!

For future reference, cooling Tite Bond glues down in the fridge/cold ambient temps is what the manufacturer recommends to vastly increase the working/open time. Learned this trick from AskWoodMan of VSC Tools.

 

[ChuckM]

I don't know about cooling the glue to increase the open time (I couldn't find anything on the Titebond site about that), but I put unopened glue in the fridge to extend the shelf life:
http://blog.titebond.com/page/how-to-store-your-wood-glue-for-longer-life

P.S. Same for OBG.

 

[luvmytoolz]

I did try the fridge once but it doesn't help me in my case as glue sessions can go for the bulk of the day, and I found putting it in and out constantly too time consuming. Plus my shed's really well insulated so doesn't get too warm at all despite the summers in OZ.

In lieu of a much longer work time, what I do need is a method to rapidly and fairly evenly spread a thin layer of glue on thin but long faces so I can get them in the clamps quicker.

To speed things up a little i'm working on a pneumatic ram at the moment for the initial and inbetween holding, and have been looking into the various hand held glue spreader tools around (like the Virutex ones), but they all seem a bit crappy really. I may end up needing to make one myself but really don't want to if I can avoid it.

 

[Packard]

I did try the fridge once but it doesn't help me in my case as glue sessions can go for the bulk of the day, and I found putting it in and out constantly too time consuming. Plus my shed's really well insulated so doesn't get too warm at all despite the summers in OZ.

In lieu of a much longer work time, what I do need is a method to rapidly and fairly evenly spread a thin layer of glue on thin but long faces so I can get them in the clamps quicker.

To speed things up a little i'm working on a pneumatic ram at the moment for the initial and inbetween holding, and have been looking into the various hand held glue spreader tools around (like the Virutex ones), but they all seem a bit crappy really. I may end up needing to make one myself but really don't want to if I can avoid it.

Re-freeze-able can cooler might work.  You would be limited to the round containers wood workers 1, 2 & 3 offer.

Note:  I have not tried this.  However, if chilling works, then this should work also.