Matthias at Woodgears did some testing on this, and found exactly what you described Chuck:
Gorilla glue sucks (at filling gaps)
The polyurethane glue is very weak if the joint is slightly gapped.
Remarkably, an ordinary aliphatic resin (yellow) wood glue actually gives slightly improved strength if the joint is loose! So there is definitely truth in the principle of not clamping too hard and squeezing all the glue out.

 

Good link Andy. I was a little surprised that the yellow glue was stronger with a bigger gap.

The idea that the more pressure you can put on a joint with clamps the stronger the joint is another woodworking myth. I see guys buy clamps because they can get 1000 lbs of force instead of one that only can exert 250 lbs when the truth is they probably don't even need 250. I read a long time ago that the most common cause of joint failure was over clamping. Over clamping causes glue starvation. The test in your link proves that more (conventional) glue is stronger.

I've had good success gluing small, difficult to clamp parts together by putting a small coat of glue on both sides and giving a few minutes to soak in and let some water flash off then just push the parts together and stick some masking tape on to hold it till it dries.

 

Not only that but it takes time to master the amount of glue to use as you will get lots of squeeze out (more like ooze out).

The one place where it's foaming makes sense is for gap filling for insulation.

 

What is the best method to apply the glue? Was the quantity the difficult part to master?

 

Very good info Chuck! Thanks for posting that.

Otis Guillebeau from Auburn, Georgia

 

I agree, it takes practice to know how to apply Gorilla glue, If one has done a poor job of creating joints and uses Gorilla glue thinking it will solve gap problems one would be correct and that has nothing to do with the bonding strength of the glue.

I've been using Gorilla glue for about 14yrs for exterior bonding in particular. I do/did a lot of work on the ocean, none of the other glues have held up like Gorilla glue. This includes segmented load bearing columns, flower boxes, trim and for a lack of an appropriate term, applications, (wood on wood build outs to form 3D designs). For some reason I it has probs with high moisture areas, cold, salt, wind, and fir wood products in one of my work areas, Woods Hole Ma.

I love Titebond, I have and use II and III, I also use both versions of gorilla glue, along with epoxy and cyanoacrylate. I'D use PL 400 if wasn't so thick and spread resistant. I've done remodel work on houses built after 1985 where PL 200 and 400 is used to lock subfloor to joists.

I have segmented octagonal VG fir columns holding up a 14X 26 roof over a patio in the historical district facing the sound since 2003, I used Gorilla glue and biscuits to align and hold the seams together, they are still together. The previous contractor used Titebond III, biscuits and nails on a north side column rebuild, protected by the Inn less than 2 yrs earlier, the seams were separating and the nails were pulling away.

It could be an anomaly but I believe the area is a mean testing zone for materials.

 

Hi Chuck

I'd say that if you have to fill gaps you maybe need to practice joint making a bit more (alright, maybe that's being a perfectionist, but it is true - wood fibres are far, far stronger than glue). Of course the original joint filling glue was hot hide glue, where you actually needed your joints to be a bit looser than the fabled piston fit - or you'd end up starving the joint. This slight looseness also allows for a minute swelling of the adjacent wood fibres before the glue sets

I started using industrial PU glue in the 1990s (Fuller), before "brand name" PUs came on the market. The main reason I used them was speed. I found that I could assemble and install stuff in tens of minutes rather than having to wait up to 6 hours or more for so-called (at the time) "fast setting PVA" (aliphatic) glues to go off. In recent times that has change somewhat and we now have some D-4 (water and weatherproof) cross-linked PVAs which set in under 20 minutes (the bottles say 5 or 10 minutes, phooey!). So the need for PUs has somewhat diminished. For high durability of joinery products made in a workshop environment, though, you still can'y beat a urea formaldehyde (UF or plastic resin) glue such as Cascamite - the downsides though are that it has to be mixed, there is a limited pot life and your cramps need to stay on for 12 hours, sometimes longer

Regards

Phil

 

Ron I agree with you about using "gorilla" type glue outside like that. It's labeled as waterproof whereas Titebond 3 is really only water resistant.

Phil you are correct about getting joints too tight. If you have to beat it together and then beat it back apart it's too tight. A tenon will scrape away all the glue on the way in. It should be a slip fit, not a mallet fit.

PVA glues have improved for sure. Clamp time is way reduced but unfortunately so has open time which is not always a good thing. Although T3 claims waterproof I don't really buy into it. I would still use a PU or an epoxy. I tried the dry powder mixed with water, equivalent to your cascamite but from a hobbyist's point of view they are too much a PITA, especially when there are viable alternatives that are easier to use.

 

I tried the dry powder mixed with water, equivalent to your cascamite but from a hobbyist's point of view they are too much a PITA, especially when there are viable alternatives that are easier to use.

Hi Chuck

As you know I come from a more "industrial" woodworking background. Certainly for joinery such as laminated spars and structural woodworking components there is just no alternative in terms of strength. The downsides, however, make them a lot less attractive to hobbyists as you have stated. Plus you need to use masks when mixing the powder (though only when handling the powder forms) and some people (very few) need to wear gloves as they can be sensitive to skin contact

Cured PU cuts pretty easy. It also sands but it isn't pretty. I try to scrape it off with a knife. That said, I've never used it for woodworking - titebond uber alles.

The biggest issue with it is that if it foams out onto an open-grain timber such as oak or ash you will never get it out of the grain unless you've already lacquered/varnished the wood. No matter how hard you sand or scrape as soon as you put a clear finish over the top that scraped-back glue foam-out just screams at you. Personally, even after 20 years of using the stuff, I've never found an accurate way to estimate how much glue I should apply to a joint to avoid squeeze-out - there are just so many variables from air temperature to air moisture content to species of timber and looseness of joint - and I do this for a living, so I've had a lot of practice using the stuff. So for work to be clear finished I'd say apply with caution - and preferably do your finishing before assembly

Regards

Phil

 

Thanks for this thread Chuck.

 

Thanks Otis and Stick.