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27th February 2018, 06:27 PM #1SENIOR MEMBER
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Polyurethane for end grain - on top of polyurethane
Hi folks,
So I unintentionally tested a gorilla glue (polyurethane) joint to destruction (was a bit ham fisted trying to remove a hot glued waste block, and the hot glue beat the polyurethane!)...
Now I find myself with a repair to do, and it's an end grain to face grain joint. On a fresh joint I would use more polyurethane, but I am wondering if the previous coat of polyurethane will have already expanded into the fibres, and it won't work a second time - any thoughts?
Would I be better off using epoxy for the repair?
Thanks,
Danny
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27th February 2018 06:27 PM # ADSGoogle Adsense Advertisement
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27th February 2018, 08:40 PM #2Taking a break
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PU isn't really designed to stick to PU. I'd be going with epoxy
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2nd March 2018, 07:25 AM #3
+1 for epoxy here. Had a similar repair a few years ago and the epoxy is holding strong (to poly)
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2nd March 2018, 07:48 AM #4SENIOR MEMBER
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If you do use PU - be sure to sand everything well. Get as much of all the old glue out as possible and re-prep the joint from scratch if you can swing it. With any glue - a new glue to old glue joint is a bad idea.
I don't want to second guess you or cast aspersions - but is there a reason you are using PU glues instead of one of the conventional wood glues? I have generally had poor performance out of PU glued joints vs using conventional wood glues (Titebond or equivalent...)
also - pertaining to the failure itself... The strength of the joint is directly related to the joint prep and how well the surfaces mated dry. The US Gov't Forest Products Group has done many studies on wood joint strength, glues, and prep... The most surprising to me was that a tightly fitted glued M&T lost approximately 50% of it's joint strength when the tenon was given approximately 0.015" (about 0.4mm) clearance on the sides.....
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2nd March 2018, 07:57 AM #5SENIOR MEMBER
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I went ahead on Wednesday night and glued up with epoxy. Looking OK so far.
truckjohn, the reason for PU initially was that it's an end grain to long grain joint (and just a simple butt joint as well) - my plan was for the polyurethane to expand into the end grain, giving a better hold than PVA (the rest of the project is held together with titebond 3).
My guess is that the reason it failed initially was my own stupidity when I was trying to break a hot glue joint, not necessarily a bad joint per say...
Cheers,
Danny
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2nd March 2018, 02:12 PM #6GOLD MEMBER
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PU seems like a great choice for end grain gluing because the glue will migrate into the timber........leaving a glue starved joint at the area where the glue was most needed. The glue that is left at the joint area, is mostly just airy bubbles which are useless as a glue. Sizing the end grain with PVA/titebond3 before assembling the joint with TB3 would have been a better approach. Sizing end grain is simply sealing the end grain fibres with glue, in this case PVA. You can dilute the PVA with water to a runny watery mixture, but I've often seen people dilute it way too much making it no different to the problems encountered with PU glue migrating up the timber grain.
generally speaking, an end grain to long grain butt joint will be fairly weak. But if that is the only option due to time or other issues, then using a glue which has some gap filling properties is highly recommended. Epoxy, urea-formaldehyde, PVA all have gap filling properties (some more than others). You need to be able to fill the gaps in the end grain timber tubes so you are not trying to glue to air. PU glue has zero gap filling properties. The only decent time for using PU glue in woodworking is for edge joining boards to make a wider top. It is good for this because it is strong enough in long grain to long grain connections, but mostly because PU doesn't creep once cured whereas PVA will creep. Some PVA's creep more than others. Using the titebonds as the example, TB original creeps the least and is pretty good for table tops, TB2 creeps a little more and is a pretty poor choice for tabletops, and TB3 really sucks for tabletops because you are almost certainly going to have raised glue lines after the glue has crept out of the joint (the glue expands and only has one way to go...out of the joint).
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2nd March 2018, 02:51 PM #7GOLD MEMBER
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As I understand it PU and PVA glues will not actually bond to themselves. The only glues that will are hide and epoxy.
If you want to size a timber endgrain to improve its bonding you have to use hide or epoxy. Any other glue will have the opposite effect and weaken the joint.
A possible way to do it is to use a basic PVA (like TB1) for the size and assemble the join before the PVA size has fully cured or if you size with TB11 or TB111 assemble the joint while the size is still wet.
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2nd March 2018, 03:39 PM #8GOLD MEMBER
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That is how I do it. The only time I ever want glue on an end grain joint is with mitre joints, even though I reinforce the joint somehow either during or after the glue up. Just before I assemble the piece, I paint some Tb1/2/3 onto the mitres and let it sit there for about 2 minutes. You can see the glue get absorbed into the timber very quickly. It isn't dry, certainly not cured, but it does stop the next round of glue that I paint onto the mitres from being absorbed so quickly which allows the glue to remain at the joint where needed rather than being sucked up into the timber and away from the joint.
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3rd March 2018, 06:32 AM #9SENIOR MEMBER
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Here's where we see the divergence from things that are sold as a "wood glue" and things that are sold as a "resin"...
Wood glues have additives built into them which "size" the surface of the wood.. Counterintuitively - they don't want "wood glue" to soak way down deep into wood fibers and pores... This is intentional for one huge reason - they don't want the glue to stain or discolor your work... You will find this is so with Gorilla glue - it does not soak way down deep into the wood... If it did, you would be screaming and yelling about ugly brown blotchy stains running up the grain lines everywhere and dry glue joints because it all soaked into the wood (a big problem with structural epoxy and polyester resins)..
Even retail quick set epoxy is formulated this way... Notice how the Box Store epoxy glue doesn't leave bad stains on wood around joints... It doesn't soak to china on end grain.. It's intentionally formulated this way...
Structural boat and aircraft resin is the opposite.. They want the resin to soak WAY down deep into the wood, fiberglass, etc for maximum wetting of the surface... Primarily - this is because of fiberglass and synthetic wovens structural mat... The strength is in the GLASS - not the resin... So for highest strength - you want the maximum amount of glass possible for the minimum amount of resin... The resin NOT soaking all the way to china ends up leaving dry, unwetted fiberglass mat - which means catastrophe.... It's nothing for a 1m square piece of plywood to completely soak up over a liter of mixed polyester or epoxy boat resin... Soak spruce end grain with boat resin and you will find that boat resin 3-6" up inside the lumber....
I had West Systems epoxy resin completely ruin a guitar top.. I was inlaying a rosette into the top. I used a liberal coating of 24hr epoxy to create a decorative ring around the rosette.... That Epoxy resin soaked 4" up into the grain of the spruce top all the way around the rosette - leaving an ugly blotchy stained shadow.... I had to scrap the whole top and start over.. The cheap Loctite 5 min epoxy doesn't do that.... But that Home Store stuff is a train wreck on glass mat... It won't soak in right and you end up using 3x as much of it to wet out your glass mat......
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