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I have some antique (100+ yrs.) oak chairs that have developed cracks in the seat. In a previous question I asked how to better support the seats. Now I am finally getting around to working on them and want to re-glue the splits that have developed over the years. Some cracks extend across the full length of the seat and these can simply be clamped and glued as any other wood slabs would be. However, some cracks do not extend the entire length of the chair (front to back) and getting the glue in the full depth of the gaps will be difficult. I have never injected glue and am not sure what the best varieties and techniques would be to get maximum contact and the highest glue strength. Any recommendations and discussions are welcome.

There is also some areas where thin splinters of wood have been lost and I would like to fill the gaps for a finished look. I have considered using clear epoxy but am uncertain what dyes or colorants would give the least obtrusive appearance and accept new varnish finish well. Again any recommendations are welcome.

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some cracks do not extend the entire length of the chair (front to back) and getting the glue in the full depth of the gaps will be difficult. I have never injected glue and am not sure what the best varieties and techniques would be to get maximum contact and the highest glue strength. Any recommendations and discussions are welcome.

If you're going to be using PVA-type glue for this (white or yellow) there are three main tricks I've seen in recent years for getting glue deep into cracks are:

  • injecting with a hypodermic (may require thinning the glue so this is the least desirable option where maximum strength is required as you might inadvertently over-dilute the glue)
  • using a vacuum to suck the glue from one face through to the other (obviously the crack needs to extend through the entire thickness of the piece for this to work)
  • wedging the crack open slightly, slipping plastic, or metal shim (e.g. aluminium from a soda can) into it and then spreading glue on the shim and pulling it through which scrapes the glue onto the inside of the crack

None of these methods will give you a complete and uniform glue film on the joint surface which is what you want, so to maximise the strength of what glue bond will form use lots of clamp pressure. Don't worry about starving the joint, this is next to impossible in a home workshop* with any number of clamps, see my Answer in Can clamps be too tight?

*Probably actually impossible in most cases as you'd exceed the crushing strength of the wood if you attempted to over-clamp.

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  • Thanks for responding. Do you have any experience injecting Cyanoacrylate glue? Or any advice regarding filling chips and gaps with epoxy with dyes, stains or sawdust for color?
    – Ashlar ♦
    Jan 3, 2017 at 3:19
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    I use CA to fill and/or bond hairline cracks but I wouldn't rely on it for any joint that requires structural strength (v. strong but too brittle). I use epoxy + sanding dust quite a lot for cosmetic fills, it's probably my favourite filler :-) I think the best previous mention is this Answer. [contd]
    – Graphus
    Jan 3, 2017 at 8:56
  • For structural filling I think it's best to mix sawing dust with fine sanding dust, but boatbuilders largely use the finest stuff only (generally commercial wood flour) and say this is strongest. You can mix to the consistency of syrup through mayo to peanut butter, the last being the most common for structural filling. In all cases, esp. if you use a mixture of particle sizes, it's very important to blend thoroughly so that all the bits are fully wetted with the glue. So probably best to use 1hr epoxy at minimum for a fill of any size so that you have enough time to mix and then apply.
    – Graphus
    Jan 3, 2017 at 9:00
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This is not quite an answer to the question as asked, but to simply keep the cracks from expanding further, you could use butterfly keys. This is a block of wood shaped like an hourglass, or a butterfly. The butterfly key is inset across the crack, then planed or sanded flush. The appearance can be more or less pronounced, depending on the similarity of the wood used for the key to the wood in the chair, or the key could be placed on the underside of the chair.

I should say that I have not used butterfly keys myself, and I'm not sure if they would deal well with the stresses encountered by a chair seat. They are more typically used for slab table tops.

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  • Thanks for the idea, but these are antiques and I want to be as unobtrusive as possible. I also need to get the wood seat to act as a diaphragm again, hence the need to get the cracks well glued.
    – Ashlar ♦
    Jan 5, 2017 at 2:40

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