Current issue of Popular WWing has an interesting article by Bob Flexner. He says that it is unnecessary to finish both sides of a workpiece, such as table top. He says that cupping is not due to the insignificant difference in the rate of moisture movement between the two sides. It is due to the more frequent application of moisture to the top side, in cleaning or wiping table tops, or due to rain on a deck. As evidence he points to the fact that the cupping is always concave, regardless of grain orientation, and contrary to what one would expect from moisture loss on the bottom. A quick look at my deck confirms that observation – heartside up or down, they all cup concavely.
Just thought I’d bring it up, as it was interesting to me.
Replies
stantheman,
Great! Wonderful!
Long ago I saw somewhere that orienting grain up down up down in a panel isn't necessary. I began trying to match grain and color instead of worrying about the grain...well...I still worried a little, but now I can stop!
Alan
That doesn't entirely surprise me, stan. The perceived and received wisdom is that both sides of a piece of wood should get equal treatment. That means a balance veneer is important in veneering and that both the top and bottom of a table top are polished about equal. Also the inside and outside of cabinet parts should be polished about the same.
The reality is that many antique tilt top tables are veneered on just the top face, and whilst it's true that lots of this type of table have fallen apart, many survive now 200 hundred years on. There are countless examples of antique cabinets still extant polished on just the exterior too.
And what's the point of polishing the inside of a cabinet with doors and drawers when the inside is for the most part sealed from changes in relative humidity (RH) by the very fact that said doors and drawers remain mostly closed but the outside is exposed to constant changes in RH?
Table tops are mopped with damp cloths and so on so there's an element of localised moisture cycling going on probably or possibly causing the upward cupping of their edges. Nowadays I lean less and less to subscribing to the 'wisdom' of polishing or veneering both sides. Slainte.
Of course, those antiqe tables were made with old-growth wood that's mostly unobtainable today, and only the good ones would survive 200 years without being junked anyway. I agree though that many antiques are finished only on one side.
I think it's going to depend a lot on the wood. A friend of mine had a large wardrobe with drawers made for him out of pine. He finished it himself (outside only, poly). When the humidity changed quickly toward the end of the summer, the carcase bowed severely.
Pete
Edited 10/7/2004 12:12 pm ET by BRADLEPC
I am not an expert in this field but because there is an effect (warpage) we are looking for the cause (1 vs. 2 sides finished) but one wonders if the finish has anything to do with it.
Wood warps without any finish so perhaps the finish is at time unrelated?
Fair enough. However things like drawer sides (no finish) and doors (finish both sides) didn't warp, and it's hard to imagine what else was "different" enough to cause such a reaction.
I generally finish everything exposed to air on large/long surfaces on the theory that the moisture content will still change, but more slowly and evenly. Seems like cheap insurance. 'Course I'm an amateur so I don't mind the extra time.
Pete
I know that good joinery is important, but I have to question how the inside of a cabinet "is effectively sealed from the changes of relative humidity" as you put it.
How to you create such a seal??
Will it hold a vacuum??
"Will it hold a vacuum??"
Ha, ha. No, of course not.
But most people keep doors, drawers, lids, etc., shut thus somewhat shielding the interior from the changes in relative humidity that the exterior of a cabinet experiences.
In the end, if a piece of wooden furniture is kept in unvarying conditions of RH the timber would eventually find its equilibrium moisture content (EMC.) That never happens of course because RH changes diurnally, weekly, seasonally, and so on and wood is of course hygroscopic.
Almost no film forming polish is impervious to water at a microscopic level either. Many pieces of furniture were and are finished on the outside with only oils and waxes, neither of which form much of a seal against water vapour.
I conclude that the interior of a cabinet is effectively sealed from the changes of relative humidity with the rider-- in normal service. And unfinished interiors that only have oiled and/or waxed exteriors might be considered more effectively shielded from short term changes in RH than the exterior of the cabinet-- in normal service. Slainte.RJFurniture
This one is fun. 10 years ago I made a bench/ blanket chest with a wide carved back from plantation grown Queensland Hoop Pine (no idea of its latin). This is the oldest piece that I have with and unsupported piece in the carcass where I know what was done. The finish is a Feast-Watson 'Fine Rubbing Oil'. The climate during construction was a typically hot-dry Canberra summer/ autumn. There were two interesting periods of differential moisture change that might support your description.
When I was carving the back (1600*250*20mm), I found that with the piece resting on the bench I had about 45 minutes before the differential in air flow between top and back caused it to cup. It took about the same time to return if I flipped it over. My solution was to prop the piece above the bench on 3 bits of 90*90 Oregon offcut to provide airflow underneath - the problem went away but I did need to keep positioning the blocks under the area to be worked.
When assembling the box the, relatively unsupported, lid started to move as soon as it was in position and lost the air flow inside the box. I took off the lid and evened out the airflow to allow it to move back, then applied exactly the same finish to both sides. 10 years later it has cupped to the limits allowed by the supports underneath (sliding dovetail across the grain). The back is still flat.
On this limited evidence, it seems that over time the differential airflow might have more impact that the finish used.
Years ago, I made solid panel pecan doors to replace plywood cheapo's in a tract home in Davis. I glued up all 20 or so panels and a local woodworker flattened then and sanded them with his wide belt.
I varnished with polyurethane and let them dry. I then came back and wet sanded and polished the front face.
The weather changed from very dry to humid with the first winter storm. Every door warped badly cupping away from the side I sanded and polished. I sanded and polished the back side right away and all the doors were flat 2 weeks later.
I believe, from painful experience, that not only do both sides need finish, that finish has to be treated almost identically on both sides so that the moisture uptake is balanced. The difference in absorbtion between the faces is small, but is additive over the entire length.
How do the rest of you feel about this informed by your own experiences?
I can't tell from your description whether the finished side of the doors was concave or convex. I could interpret "cupping away from" either way.
The door cupped away from the sanded and rubbed out side. I think that was because the finish was thinner after sanding and rubbing and the surface was maybe "violated" in a way that made it more permeable.
There is no question that panels that are not completely supported (doors, some tabletop, etc) do benefit from being finished on both sides. However, carcases and other things that are supported with cross boards and table skirts, so not require finishing on both sides.Howie.........
And that would fit with things like drop leafs on tables, which notably DO warp often.Gretchen
Excellent point.Howie.........
My feeling about finishing both side either with lacker, veneer, paint etc. is that the finish itself applies forces on the wood surface that should best be equal on both sides. I once painted a solid plywood door approx. 1-1/2" thick on once side only with solvent based paint. It cupped about 3/8" (over a width of 35"). After applying the paint to the other side it almost returned to flat. I found it incredible that that such thick material could be bend so much. Another experience is a 3/4" particle board table with thick melamine sheet ('Formica') bonded with contact adhesive. It cupped badly over time.
I make a lot of small to medium sized boxes and seldom, if ever, finish the interior of the box. In fact, most have a thin veneer of either aromatic cedar or spanish cedar on the interior. Haven't experienced any problems yet (5 years +).
Some people claim that age or high temperature drying makes wood unsuitable for bending. I suspect either will also prevent wood from warping due to moisture changes.
Even my kiln dried wood sits around for years before I make furniture from it. It seems to stay flat regardless of how it is finished.
I think it also depends on what type of finish you use. Some allow vapor to pass more easily than others. Put a tight water base on it and you are probably asking for trouble. I have also seen the effects of unballanced finishing.
Mike
Perhaps he's correct but I'd still finish both sides.
I've seen many, many manufactured pieces (fairly high-end) that only have one side finished.
I'd do both sides if for no other reason than to keep a project from looking like it was hurried down the assembly line.
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