AWGB Woodturning Forum
General Category => General Discussion => Topic started by: willstewart on June 25, 2020, 03:07:49 PM
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I have often on various woods used sawdust made into a paste with wood glue (eg Titebond) as a filler for cracks etc. This seemed fine until I just tried it with oak. The paste looked fine and a good match but it went black on drying. The colour seems all through and does not disappear on sanding the surface. Has anyone else seen this? Does all PVA do it? I seem to recall that oak is the traditional base for some inks.
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You may find it is the tannin leaching out of the oak fibres?
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It could well be. The glue directly on the wood has no effect so it must be something to do with the powdering.
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Where did you pick the saw dust up from. Was it from a bench near a grinder which had filings on it? I have used oak sawdust mixed with PVA and did not have the reaction that you got.
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Made specially from an offcut of the same wood. Hard to see why there should be metal in it - though I did use a stainless steel knife to spread it.
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Did you use a metal tool to mix the paste or spread it? It only needs the smallest amount of iron, and the moisture of the glue on the metail can get that, for it to react with the tannin in the oak. Another possibility is if you used water to make the paste more workable, as that may well have iron compounds in it. I had a similar problem when making an oak table with a customer. He insisted on wiping off the excess glue with a damp rag (In my own workshop I let the glue go off then remove any excess with a paring chisel). The result was black staining all along the joint lines.
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I think you may have found the cause. I just noticed you posted that you used a steel knife to spread it, while I was typing my reply.
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well there's a lesson for us all. I'm sure I wouldn't have thought of the spreader causing problems. In case the idea is new to readers I have been using the wooden knives on offer in Greggs to spread glue with. Works a treat..... and they're free!
Brian
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Thanks all! Very useful feedback, I think you have it and I will be much more careful in future. Fortunately in this case (a long case clock) the filler is minimal and I can arrange for it to be behind out of sight.
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A few years back I sanded and refinished our oak parquet floor. I bought some resin to mix with the sanding dust to make a filler to fill any gaps between blocks. I mixed a small sample using a metal spatula and the mix went black so I mixed another batch with a plastic mixer and applied it with a plastic spreader and it stayed the right colour - lesson learned! :-)
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Paul - and me. It is impressive that even a stainless steel knife has this effect (& unfortunately not straight away - only as it dries).
Of course small dark regions around old blacksmith nails in old house boards are common (though I assumed some of the boards were used green). But it does make one wonder about steel turning tools, especially on wetter wood.