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I've recently looked up some info on PVA wood glue and found the following:

http://www.constructionchemicals.co.uk/blog/2015/07/13/what-are-d1-d2-d3-d4-adhesives/

These gradings are part of the European standard BS EN 204 governing the classification of wood glues for non-structural applications.

There are 4 main types of durability grading, running from D1 to D4. The differences between these are listed below:

D1 = Interior areas, where the temperature only occasionally exceeds 50°C for a short time and the moisture content of the wood is 15% maximum

D2 = Interior areas, with occasional short term exposure to running or condensed water and/or to occasional high humidity, provided the moisture content of the wood does not exceed 18%

D3 = Interior areas, with frequent short-term exposure to running or condensed water and/or heavy exposure to high humidity. Exterior areas not exposed to weather

D4 = Interior areas with frequent long-term exposure to running or condensed water. Exterior areas exposed to weather

(BS EN 204 is a British/European standard.)

My question is: For painted exterior joinery (hardwood, with microporous water-based "breathable" paint), exposed to weather (windows and doors etc.) would D3 be acceptable, or must D4 classification be used?

I know it says that D4 must be used if exposed to weather, but presumably the paint protects the glue such that it is not directly exposed? On the other hand, the paint is breathable and so technically the glue joints ARE exposed to humidity.

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Well, I use Tite-bond glue product and they have three grades (it is also PVA glue, like your talking) original, water-resistant, and water-proof.

Being that your paint is breathable I would think moisture will get to the wood. Original would not be your wise choice. I use water-resistant in all my projects, even if they have nothing to do with any moisture, besides the fact that some projects have polyurethane over them as well.

But being breathable paint, I feel that water will get down into the wood, and could stick around for a while, because the moisture has to travel back out of the paint. I could see water-resistant not holding up. Not saying it would last, but I don't think its worth using.

For your case, I feel that using water-proof would be the best bet. Simply because it should last the longest. On the bottle, even though it is water-proof, does list that it should not be submerged in water for LONG periods of time. Obviously, this is not the case but wanted to say that because it does suggest this will, at some point, wear out.

Simple answer from me, Water-proof glue. My opinion, it is the safe bet.

  • Did not add PVA glue grade, looks like you found a good source for that. – Ljk2000 Oct 25 '17 at 13:43
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The standard pretty much says it in plain English, D4 should be your only thought here if you go with PVA. There's no advantage in skimping a bit to save just a little money for a glue that might do you when you can can go with a glue that is purpose-made for the application.

PVAs are not the only option though
Polyurethanes, urea-formaldehyde (e.g. Cascamite) and epoxy are all viable alternatives. Any of the three will tend to be more expensive than even the pricier PVAs, but they are worth considering if some aspect of their working characteristics1 or performance2 might be useful.


1 Longer open time (sometimes much longer, an hour or more if necessary), no 'grab', lower working temperature etc.
2 Any joint subjected to continuous heavy loading shouldn't be glued with PVA. Ditto if the glue needs to fill any gaps PVA is a very poor choice (as is the common moisture-curing polyurethane glue because although it foams up the foam has no strength).

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BS EN 644:2003 "Timber Windows - Factory assembled windows of various types" (which is the main British Standard for timber window design, construction and finishing) says:

Adhesives shall conform to BS EN 204:2001, type D3 or BS 1204.

So it seems that D3 grade PVA is the accepted standard for timber windows which will be coated with a paint or stain finish. It could be that D4 is meant more for fully durable timbers like Oak which may be left unpainted and fully exposed to the weather - but I'm sure it wouldn't hurt to use a higher grade than strictly necessary.

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