Does painting/varnishing affect wood movement ?

[Quickben]

Hi all,

I'm looking to get into furniture making and still learning the basic joints etc, I'm picking this aspect of woodwork up nice and quickly. However, I know nothing about wood movement and how to predict and therefore combat against it.

Does sealing timber with paint or varnish etc, go towards reducing movement at all ? Is movement purely down to a relief of tension/stress or is it due to the wood drying out ?

Or a combination of both ?

 

[Orraloon]

Wood movement is mostly due to it expanding with water or shrinking as it dries out, so it does both depending on conditions. How the board is cut from the log also affects how the wood will move. In general quarter sawn is best. Some timbers have very small shrinkage while others can be quite large so wood choice comes into it. It's going to take quite a bit of reading so head for google. Here is a starter.
http://www.popularwoodworking.com/trick ... -expansion
Regards
John

 

[RobinBHM]

Finishing wood with paint varnish etc slows down the movement of moisture but does not stop it.

If you painted the piece of wood on all edges and placed ti outside, it would slowly but surely gain equilibrium with the ambient moisture content of the surrounding air.

 

[Phil Pascoe]

Yes - which is why you should always paint/varnish/polish/oil both sides, otherwise one side will absorb moisture and dry out quicker than the others and move differently.

 

[custard]

Like Phil I almost always finish both sides, but I've seen plenty of antiques that were only finished on one side without adverse effects. And when I'm making case work with an oiled finish (cupboards and the like) I tend not to finish the interior because the smell can linger for months and even years. Again, no adverse effects, and I've made hundreds of such items over many years, so if there was a problem I'd have encountered it by now!

Another thing to bear in mind, finishes may slow the transfer of moisture vapour, but apart from a thick layer of epoxy you'll never actually stop it.

 

[profchris]

Once wood has reached equilibrium (ie properly seasoned and allowed to adjust to its environment) it doesn't "dry out" any more.

What happens is that if the atmosphere becomes more humid the wood absorbs moisture and swells. If the air becomes drier, the wood shrinks.

Almost all this movement happens across the grain. If the grain is vertical the wood just gets wider or thinner. If the grain isn't, then the wood will tend to curl up widthways - if you look at the end of a board you can see which way it will move.

So you need to plan for this when choosing wood and constructing something. The obvious start is to move the wood into the environment where the finished article will live, let it acclimatise, and then work it to shape. If the environment has stable humidity, nothing should move much.

If you can't do this, design to cope with movement. A tabletop in a conservatory will move, so you could arrange the grain so the sides where boards meet move in the same direction, to minimise ridges and gaps. You'd use a sliding fixing for top to frame to allow the top to get wider. Ideally each board would be vertical grain.

Sealing one side is only a problem for sudden humidity swings, and will accentuate the problem. Eventually though the wood will stabilise to where it would have been if unsealed. Until humidity changes again!

 

[ED65]

Does sealing timber with paint or varnish etc, go towards reducing movement at all ?

To some degree yes, but there are a couple of big buts. The type of finish and how thickly it's applied are both factors and oil finishes and wax have so little effect it's irrelevant. Film finishes (shellac, varnish and lacquer mainly, but also paint) on the other hand can slow water uptake and loss significantly, enough that they can help in lessening movement through the seasons and therefore stabilise furniture. But the catch is that film finishes have to be applied fairly thickly and uniformly for this, especially the end grain which is many many times more porous than face grain – up to 100 times difference!

What this boils down to is that unless you're willing to fully encase your furniture in a fairly significant coat of a film finish, paying special attention to the end grain, you have to still allow for movement, almost as though the piece were left unfinished.

Is movement purely down to a relief of tension/stress or is it due to the wood drying out ?

As already mentioned it's both water uptake and loss, as the humidity in the environment changes relative to the moisture content (shortened to MC) of the wood. Wood we consider dry can still shrink further if taken to a drier environment, which happens to antiques bought here and brought to the US frequently.

 

[Mike Jordan]

You don't combat timber movement you make allowance in your design to allow it to happen without spoiling or destroying the product you make. Many traditional designs take account of movement and allow it to happen, a solid table top fastened to the frame with buttons will allow an increase in the width of the top without breaking the joints in the frame. Shrinkage of the top can also occur without the top splitting.