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I am looking to purchase some unfinished ash strip wood for some interior decorative purposes. The retailer advised that whilst it is 'ready to use' (in reference to the wood having been planed etc), it is still bare wood and thus could be prone to warping, e.g. twisting.

Have I understood it right that such warping is caused by the moisture leaving and entering the wood? With this in mind, would I be right in saying that I could mitigate the issue by applying varnish?

I am also looking to apply a wood stain beforehand. Would this be a variable that could affect any tendency to warp?

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  • "bare wood... prone to warping and twisting." This is a little nonsensical. Both bare and finished wood can warp, but wood prone to warping may be poor wood to begin with, and/or was dried badly. Look inside your attic at the rafters etc..... see any finish? The details matter, so can you specify what you're doing? "I am also looking to apply a Wood Stain beforehand." What exactly? It makes a large difference what you use here, and unfortunately the "wood stain" category in finishes has many completely different products in it, some of which aren't actually stains by definition.
    – Graphus
    Nov 20 '20 at 9:34
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    Sorry in case it's not clear, I'm asking what wood you bought & what you're intending to do with it. We can't tell if you bought stuff already milled to shape — various mouldings or trim pieces etc.— or whether you've bought boards that you intend to process further. There's a huge difference in how you typically treat the two things, not least because thin or small-section trim/mouldings aren't prone to bad warping in a way that can't be overcome when they are attached in place. But on the other hand a warped board from a mill/lumber yard will typically be made flat again during final shaping
    – Graphus
    Nov 20 '20 at 9:44
  • Sorry only saw that you're in the UK after I'd posted. So please substitute timber merchant for lumber mill :-)
    – Graphus
    Nov 20 '20 at 10:09
  • @Graphus: I was informed by the Retailer that the wood could be prone to warping, due to it being a Bare Wood. No other information was provided to me, on that matter. You make a good point, regarding the rafters.
    – Craig
    Nov 21 '20 at 2:25
  • @Graphus: I am looking to purchase 30mm by 30mm Mouldings/Beams, so the wood is fairly thin. It's purpose is decorative. I am looking to use the Wood to frame interior doorways.
    – Craig
    Nov 21 '20 at 2:31
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Varnish will likely help but it won't completely negate issues with warping and twisting. There are many other things you can do to help:

You are correct in that the key to stopping wood from warping is stopping moisture from leaving or entering the wood too quickly - wood shrinks around 1% in its width for every 3% change in moisture content (IIRC) and moisture leaves through the end-grain around ten times faster than it does from the sides of a board. Drying (or increasing in moisture content) unevenly will cause one part of the wood to shrink faster than another which causes warping. Therefore, one of the largest differences you can make to the stability of wood (short of something like chemical modification of the wood e.g. acetylation - not practical in a home setup) is to seal the end-grain - varnish or paint is not enough - you will get the best results from a purpose-made end-grain sealer which is usually applied as a separate operation before finishing with a paint or varnish. Again this seal will not completely stop moisture transit (there is some very tiny amount of evaporation even through a sealed glass container, after all) but it will do a great deal to slow it and this is what we want - nice gradual adjustment - which helps it to be more uniform across the piece also.

Generally, be careful about how you machine the wood. The surfaces of a piece will always be a bit drier than the inside, unless the wood has been in a stable environment of temperature and humidity for a very long time. If you are to plane or cut the wood down in thickness or width then you should try to take an even amount off of each side, otherwise you can cause uneven drying of what were the inner parts of the timber (which have now become the surface), which may cause cupping, twisting etc.

Similarly if you cut grooves into a piece it may lead to cupping (the groove will typically want to close up as the new surfaces dry and shrink) - I don't know that there's too much you can do about this but if you machine a groove, knock a few blocks the same size as the groove in, every few inches, and allow the wood to adjust in moisture again, with the blocks in place, slowly, before removing the blocks and continuing.

If you're able to allow the wood to acclimatise to its end-use environment, slowly, before processing, and then you're able to keep that environment at a stable temperature and humidity when the end product is in use, this will help with stability. Just by virtue of being used internally, rather than externally and exposed to the elements, the wood will be more stable (assuming that your interior environment is at least somewhat controlled in temperature and humidity).

Another thing you can do (but is dependent on your usage) is to laminate many timbers together with glue - again this will typically increase stability, through the averaging out of defects and twisting/warping forces, but will not always completely solve the issue.

To make it even more complicated, the type of cut (as in, how your board was cut from a log) will alter its stability characteristics, as will other things like how the grain is (straight, wavy, interlocked, etc.) so it can be a real minefield.

So in summary it's really a combination of:

  • How the timber was cut from the log.
  • General characteristics of the grain etc. of the wood.
  • How the timber was dried from the mill (how quickly, to what moisture content etc.).
  • How it has been machined after drying.
  • How it has been sealed, particularly the end-grain.
  • Service conditions.

Typically, a stain or other finish will not cause much in the way of warping - so long as you apply in a thin coat (as opposed to say, dipping and soaking the timber!) and have sufficient warmth and airflow, it will (substantially, if not completely) dry within a few hours and the wood will not uptake a significant amount of moisture from it in this time. Don't be tempted to use a high heat to dry your stain or varnish more quickly - this can cause more problems than it solves.

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  • Many thanks for your comprehensive response. Just to confirm ... Sealing: Am I right in reading that it is not possible to achieve a 100% seal. The best one can hope for is to keep any exchange of moisture to a minimum, where such exchange is done at a slow rate. Stability: This slow rate of moisture exchange, then ensures that moisture levels remain consistent throughout the piece of Timber; preventing any distortion to the end-product. (1/2)
    – Craig
    Nov 20 '20 at 3:28
  • End-Grain: The current end-grains are achieve through a Mitre Cut. Would it be fair to assume the moisture exchange would be reduced, from circa 10x, if the Mitre Cuts were joined? End-Grain Sealer: Can I apply this end-grain sealant to the entire piece of Timber, or is it specific to just end grains? The Process: Have I got it right that the process is: Seal Grain > Apply Wood Stain > Apply Varnish. Obviously leaving each step to dry first. (2/2)
    – Craig
    Nov 20 '20 at 3:28
  • @Craig, essentially yes, the term 'sealing' is bandied about a lot in wood finishing but almost never does it mean actually mean what the word implies, sealing the surface 100% or close to it. Exceptions are marine uses and things like bartops. Re. end-grain sealer, in general this is for green wood and rough-sawn lumber only. It's not much used in any other context, and has zero applicability to finished pieces which is what I half suspect you're working with here. If that is correct it's important to mention that it's extremely common for the end grain not to be sealed at all.
    – Graphus
    Nov 20 '20 at 9:53
  • @Graphus: Thanks for your input. The only information I have is that the product is an Ash Bare Wood. The retailer has stated that the wood is all cut and ready to use, as is. Just that it may be prone to warping due to it being a bare wood. Aesthetically, it looks finished. I guess it just needs that final layer of protection against warping.
    – Craig
    Nov 21 '20 at 2:20

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