I have read (and followed) instructions that when fitting biscuit joints, that PVA glue should be used, as this as well as gluing, due to water content , forces biscuits to swell.
I now tend to use Gorrilla Polyurethane glue on external work rather than PVA, anybody know if Polyurethane glue is suitable for use with biscuit jointing ?
I have used it succesfully joining two halves of an outside table top last summer. It's been outside since then without problems.
Jonathan
It'll work. You should be using plastic biscuits for external work anyway, so swelling is out of the question.
I don't consider PU glue fit for anything. Pain to use, poor in service. Pitiful outdoors.
I certainly wouldn't use it for biscuits. One school of thought is that biscuits exist solely for aligning work while the glue of your edge jointed butt cures. Once cured, the strength is almost entirely from the glue joint, not the biscuit. So use them with a glue that gives you quick results, especially some swelling, rather than PU glue that takes ages to achieve any strength and will absorb water from it rather than supplying it.
Which PUs have you had such poor results with?
I did some sample glue-ups a three or four years back and left them in my garden. The D3 PVA gave up a year or two ago, the D4 PU is still going, as is the cascamite.
Plastic biscuits? Never seen or heard of them. If moisture ever gets as far as the actual biscuit, then the joint is buggered anyway.
So what if you haven't seen or heard of them?
Why use a durable exterior timber and stick a non-durable timber biscuit into it?
Ever seen how fast beech spalts?
Similarly, Gorilla glue is still holding up a somewhat odd cross halving in a little picket fence of mine (fence post has to be off the fence line because of an obstacle)
It means they are not in common use.
For water to reach the biscuit it would have to penetrate the glue line. If water penetrated a waterproof glue line the joint would fall apart whatever the biscuit is made from.
Irrelevant.
I've used polyurethane glue with biscuits when edge jointing boards to make up stair treads.
I was taught that to achieve high strength PU glue joints requires the faces to be very closely matched and a good deal of clamping pressure as the glue goes off.
I did a test sample with offcuts of the boards I was edge jointing along with a single biscuit - then destruction tested the results with a big hammer - I was more than happy with the result.
I did the same and the wood came apart before the glue.
Biscuits are not commonly used in exterior timberwork - at least by anyone who knows what they are doing, and if that is the case, they will use plastic ones. I think you might be digging a hole deeper...
It isn't about joint failure, it is about adding a nice little source of 'rot compost', and it is about moisture not about 'water' in the sense you are thinking about it.
Erm, OK. "Yes it is, No it isn't". Care to explain to me (assume I am a bit stupid or something) exactly why it doesn't matter for exterior joinery that beech is very very susceptible to rot in the presence of moisture (much,much less durable than a nicely resinous pine for instance)?
Balcotan mostly. Many of my problems were caused by Robbins selling old stock that was already getting crunchy in the tube. I've also used (and disliked) Gorilla, although at least that's a low-foam and isn't as messy in use.
Because it's a disposable alignment widget. No biscuit is appropriate for long-term outdoor reliability.
The only plastic biscuits I recall seeing were self-locking barbed ones from Lamello (to avoid the need for clamping) and were a hellish price. They certainly weren't intended for outdoors.
Kind of contradictory there, given you can't get the disposable thing out after making the joint. I agree with the second sentence, and I wouldn't personally use any biscuits at all in exterior work (and rarely use them for anything other than quicko mdf stuff, but having planos helps there). What I am saying is if you want to use biscuits for alignment of a long grain glue joint for exterior work, use a plastic biscuit, not a beech one.
There are others around - Knapp do (or did prior to their going into administration and being bought out) a few types designed for wood, and there are others for use with solid surfacing which will suffice. The Lamello one was not designed for that purpose (and yes, they are pricey) but it'll fulfill it.
I think that's what I was using at the time I did the samples - I now use cartridges as they last better.
If you are talking about pine, even untreated it is more durable than beech, which is about as undurable as a wood can be. You can't prevent wood from taking in moisture with a (normal) surface coating.
OK.
Festool sell interior (beech) and exterior (african mahogany) dominos, by the way.
Clearly they've failed to grasp the wisdom of handyman.
Impermeable wood? Very impressive stuff - is it made of plastic?
OK, why so aggressive about it, then?
Axminster sell both the lamello types (the gripper ones, and the solid surfacing ones, or did. I haven't seen the Knapp ones for agres, I think either Ney or Scott & Sargeant used to carry them.#
Maybe you will take notice of 'professional deck builder' mag:
I bet you were a knock-out at primary school.
Thanks for various comments .... I also contacted biscuit manufacturers, Biscuit tool makers & glue manufacturers
Mixed results.
All bar one said that PU can be used with biscuits, several added that recommended technique is to dip the biscuit in water before inserting into joint, and spraying over with water before clamping up (normal PU use advice).
Goriilla glue said absolutely that PU can be used ... Pointed to following test ...
Plastic biscuits do exist (never used them) for example :
But only for joining "composite manmade materials, such as Corian® or Varicor® etc."
Not exterior timber.
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