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Thread: Glue on top of glue
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22nd November 2022, 03:23 PM #1
Glue on top of glue
I may have unconsciously picked up the mantra that glue doesn't stick to glue somewhere along the lines. E.G. Old joints needed to be completely cleaned before any reglueing attempt, maybe unless you're using hide glue.
Today I needed to reglue the spine of a so called "perfect" bound book (ROFL). Anyway, an apparently perfectly acceptable way of doing this is to build up the glue binding by applying multiple layers of acid free PVA glue, waiting for each layer to dry before applying the next.
So Bookbinders believe PVA glue does reliably stick to itself. So it got me thinking - why does the woodworking fraternity seem to recommend against this practice when refixing loose joints? Is the WW PVA glue unstable because it breaks down from some acidic reaction? If so why use WW PVA glue at all?
I actually used Elmer's School glue for the job, but I'm wondering what might happen if I used the Titebond 3 from the workshop.Franklin
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7th January 2023, 09:55 AM #2GOLD MEMBER
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I dont know this for a fact, but I used to keep glue in a old sauce bottle with no lid over the tip, I used to get a small square of off-cut timber, squeeze glue onto it and leave it on top of the tip so the PVA sticks to the tip, over time the glue would build up into a thick layer of glue that seemed to be bonded to itself pretty good. However, the mechanical properties of said dried glue mess would be pretty weak I think.
edit: also, I seemed to remember people applying PVA or watered down PVA onto end grain, letting it dry, then adding another glue and joining as a way to increase end grain bond strength
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7th January 2023, 10:22 AM #3
It's all down to mechanical strength. Take two pieces of wood (preferably not soft-as-crap pine) and glue them to a piece of cutting board or something else that doesn't stick; use clamps as normal to ensure the bond with the wood half is as good as you can get. After 24rs break the joint; then glue the two pieces of wood together on those glue joints. Clamp as usual and then break the joint afterwards; it will break at the glue line as that is now the weakest part.
You get the same effect on joins where there is no close contact between the mating surfaces and the PVA ends up being used as a gap filler. Circular saw and jointer/thickness scallop marks reduce the contact area which is why cabinetmakers often use a handplane as the last step in edge preparation.Nothing succeeds like a budgie without a beak.
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