Hi Gents,

It's probably a difficult property to quantify, but I'm curious as to just how much upwards - or outwards - a typical polyurethane adhesive will actually foam when setting?

Selley's, for example, claim that their "Durabond" has "gap-filling" properties. I don't know what the stuff is like on a vertical surface, but for whatever coating-thickness can be reliably achieved on a vertical surface before serious "running" occurs, how much will the stuff typically puff outwards by?

And also, if both surfaces are coated with it before they are brought together, will the stuff behave like a Contact-adhesive? (This should effectively double its gap-filling capability...)

For what it's worth, the application I have in mind is the fixing of some VJ-look MDF panelling to some wall-studs that are no longer exactly straight. Some of the studs are "high" and "low" relative to each other, and "curving" inwards and outwards into the bargain (which makes simple "packing" of their front faces a bit of a pain...)

Methinks some type of foaming adhesive would be a good way to go, but it would have to be able to do a decent amount of gap-bridging for its part. As you can imagine, the MDF sheets aren't all that interested in flexing in and out to suit the studs...

Some opinions regarding which of the polyurethanes are nice and thick (so as to limit the amount they will "run" on a vertical surface) would also be very handy, as well as any suggestions regarding other types of foaming adhesive that might be suitable for the job.

Many Thanks,
Batpig.

Hi Batpig

My experience is with Titebond PUG. How much foaming depends on the amount of excess and the available escape gap. For example, if I have a blind hole into which I tap a nut, the thread hole can become completely foamed up. This glue is $30 per 355ml from Carbatec, so perhaps there's a cheaper brand. However, this is very strong glue, but conversely the excess is very easily trimmed off with a chisel etc.

No, it does not behave like contact. I forget the working time, but it begins to foam almost immediately.

I guess the thing to do is to buy a bottle of your choice, and do a test with it on a vertical surface to see how it runs.

Summary - good stuff, and a welcome addition to any workshop for a huge variety of gluing applications - get a bottle anyway.

The other thing is that if you're going to screw the panelling in place you could use long threads which will very effectively bridge the gap.

Regards, FF

G'Day Batpig,
I've used the Durabond and found it almost will stick anything together.
I've found I only need to use about 1/3 the amount of other glue with the expanding.
I've even used it to fill a gap 8mm wide and while it's filled the gap, I can't vouch for it strength.
The original demo I saw on TV with Durabond was to use the glue to fix door hinges to the door frame and then hang the door; so it would seem strong enough.
I hope that helps.
Cheers, Crowie

I've used this product quite a bit. In answer to your questions; it will expand by at least a factor of ten with moderate humidity, but in drier conditions perhaps only x5. Because it's a moisture curing product is sometimes pays to moisten the surfaces before application. It is also very runny in the vertical position until is begins to cure which can take anywhere from 5 to 20 minutes, again depending on the humidity.

My experience has been that it doesn't produce as strong a bond as a good yellow glue in a tight joint, but in a less than perfect joint (with gaps of 1mm+ for example) it's far superior to water based glues. My favorite applications for this are mortise and tenon joints which have a loose fit.

I don't think it's well suited to an application with such an open joint, it will tend to run away.

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I think you're barking up the wrong tree. If gaps are too big for stud adhesive they're certainly too big for polyU. Also because it foams it might push the sheet out of true. You might think that it should relieve the pressure by foaming out to the side and that is what happens to an extent, but as the foam skins it then tends to act like a pillow and puffs up. If you think of the process with polyU, you have to install and hold the sheet true until the polyU sets and stops expanding. If you have to do that, you might as well do it with stud adhesive and avoid the other complications. Typically I do it by using a straight edge to determine the packing material required and carry a range of different thickness shims to pack, and only pack at positions where you are going to fasten with nail so you don't need to spend a lot of time doing it and you don't need to true the whole stud. Between those packers put your stud adhesive walnuts.

Cheers
Michael

Dear Gents,

Thank-you for all four brilliant replies .

FenceF - I will definitely get a small bottle of Durabond from the BigB, because going by what Kman has said, that sounds like more than enough expansion for what I've got in mind. Strength sounds good too, Crowie. I know that the expanding polyurethane-foam sticks like sheep-dip to anything you spray with it, and wouldn't have far short of the positional stability that I'd be needing for the job, so the adhesive-version has got to be better again for stickiness and stiffness. There's certainly a few different experiments that I could try with a bottle of the stuff.

Michael - what you have said is definitely food for thought and has most certainly been on my mind, because the original conception was to spot-tack the MDF to the high-spots, and then bond it properly to the frame with pumped-in insulation-foam from the other side. Popping-off was always going to be a risk in those circumstances, though, so that's why I've swung over to the idea of a dedicated adhesive on the front of the studs only (because the excess foam would have at least some sort of escape route - as you have already noted...) I've also got the idea in the back of my mind of using bugle-head screws as "height setters" that I can align with my straight-edges, prior to the glue-up and fitting of the sheet, so these would give me something to push against from the sheet face with my straight-edges during the curing phase, and would spare me from having to "pack". But I'm still turning over the whole concept in the back of my mind, so everything remains possible.

Many Thanks all,
Batpig.

BTW, in my experience, the foaming doesn't push anything out of the way - it tends to follow the line of least resistance.

I agree with Michael - I can see nice big bubbles of expanded glue peeping out from the bottom of the sheet onto the floor.... I think you'll be in for a messy job with unpredictable results. This glue runs everywhere - using it vertically in a gap is the last thing I would do with it....
Good luck though!

Lee

Dear Lee,
I hereby do solemnly swear that if I proceed with the use of polyurethane adhesive for the said application, that I shall conduct appropriate testing beforehand to determine the suitability of the said product for the said application, viz the potential bulging of sheets and running of adhesive...

'Sides that, I've got a "Clampish"-sort-of idea in my mind that would forestall any potential bulging if I do go down that route, as well as another "Steel-Studish"-sort-of-idea that involves the use of polyurethane foam instead.
Nothing firmed up yet, though; I'm still thinking it all through...

Best Wishes,
Batpig.

G'day , I'm not sure 100% what your job is but some type of decorative vee joint sheet material, is it to chair rail height or full height?

But no matter, I would drop the glue idea completely and run with split battens, that way you can get a nice 'true' wall ie. Completely flat and will not encroach too much into the room (12.5 more than your glue idea, which in my humble opinion is doomed)

If you would like any tips on how to do this no worries .

__________________
darren stewart

http://oldschoolcarpentry.com.au

This is how we do it on boats:
Put some packers where they need to go. (get your corners right, your joints and maybe one or two in the middle to keep the sheet flat) you don't need to put them everywhere where you would have put a fixing since you are glueing.
Then use a thick not running glue. I use sikaflex and put little blobs on the studs then just push the sheet on and the suction of the sikaflex should keep it in place. Same thing can be done with studadhesive or liquid nails, you then just need a way to keep the sheet in place till the glue cures. An option to keep it in place I use is to put some blobs of car bog here and there since this cures in a few minutes.
IMO expanding pu is for joints that are meant to be closed but have some gaps, not to bridge gaps to start of with. I'm sure you'll find out in your test pieces.
Good luck