Good question. Given that it doesn't actually "varnish over" the fine
surface features (like polyurethane would) you should still get some of
the mechanical keying. On the other hand, since it renders the surface
hydrophobic I'm sure it would reduce the effectiveness of PVA type
adhesives (where, presumably, surface tension helps to draw the
particles of emulsion into the surface). I guess polyurethane adhesive
*will* wet the relatively oily surface left by creosote, but the
creosote will presumably fill some of the porosity that would otherwise
help the adhesive to "key".
Even if it does not provide much tensile strength to the joint, its gap
filling qualities will provide some shear strength but I'm sure you are
right to add some nails or screws.
There's some advice here, just not for treated lumber.
And the conditions of the test are pretty ideal.
It's a furniture wood test.
On Friday, 24 July 2020 21:22:40 UTC+1, Paul wrote:
I am not in the least surprised by polyurethane doing so poorly.
It can be useful but it isn't strong. (I use it for wall plugs into crumbly walls and oversized/misshapen holes. Make the hole and dampen it. Coat and fill the plug with PU. Let it set. Cut off any excess that has foamed out. Then screw into the plug.)
Shame they didn't include any of the softwoods. Quite possibly their differences are so great that the bonds might have been very different.
And they didn't test Aerolite or similar.
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