While both highly respected, Bruce Hoadley and Bob Flexner have contradicting views when it comes to finishing both faces of a panel. Hoadley says it is important to finish both faces to keep the panel (moisture exchange) balanced, but Flexner calls it a myth, claiming it is unnecessary. Who is right?
From my own anecdotal experience, I wonder if it depends on the finish used. I’ve never had an issue with warping or cupping when using an Oil finish, however I consistently experience these issues when using a Polyurethane. Some finishes retard moisture exchange better than others.
Thoughts?
Replies
Flexner has been finishing wood his whole life. His advice is tgge same as my experience.
Antiques were never finished on the inside, and never had a problem. It wasn't until factories started spray finishing furniture that it became more difficult to keep finish off the inside than to just spray the whole thing.
Both Hoadley and Flexner are considered experts and widely respected. Both of their books were required reading way back when I was in college.
Regarding Antiques, I wonder if the reason they didn't have any issues with warping/cupping was because the common antique finishes (Oil, Wax, Shellac) weren't all that resistant to moisture vapour, so the difference between the finished and unfinished surfaces wasn't enough to cause a problem, where as modern finishes (lacquer, urethane, epoxy) are far more resistant and reduce the vapour exchange enough to cause a problem.
I recently finished a panel that had been glued up and sitting in my shop for a few weeks without any signs of movement. I coated one face with Linseed oil. Flooded it on, let sit for 30 minutes and wiped off. Two days later and still flat, I applied a brush on coat of Minwax Polyurethane to the same surface. I checked on it the next day and found it had cupped significantly. Humidity outside and inside the shop was unchanged.
I've absolutely had panels cup when one side was finished.
In my case it's been warm and humid when it happened. Basement shop without AC.
Flexner may be correct, but I wonder if he always has good air conditioning.
Mike
My experience of 14 years designing and building custom furniture left me with a couple of "golden rules". The first one is "Whatever thou doest to one side of a piece of wood, thou shalt also do to the other. Always." (I am also a retired minister.) Following that rule cost me little in time and materials; failing to follow it cost me significant amounts of time and expense to compensate for inevitable wood movement.
-- Mac
Oh geez, what's next a sharpening thread? LOL
Finish does NOT prevent movement take it from there!!
Well said!
This could be argued like tails or pins first. Method of construction? The last antique I worked on was made about 200 years ago. My work was mostly done on the top panel. Two pieces of Cuban mahogany. Each about 12 inches wide. The sides were about 1/4 inch thick and the centers were 1/2 inch thick. You could see a gentle curve to the growth ring running from side to side of each board. The top was split open in three places. The center on the original glue joint but just in the center, ends were still tight. The two other were near the center of each board on a growth ring. Top was a oil finish underneath was four boards hide glued opposite of the two top boards.
Pins. No, wait...tails! Now I'm all confused.
Two ways of looking at this:
1. You "pin" the "tail" on the donkey, so its obviously pins first.
2 What would you rather do, set your pin on a tail, or set your tail on a pin?
If you don't believe me ask Frank Klausz.
For me, the best info is long term stability studies. I'd examine 100 year old and older furniture and see what was done and how well it has stood up. If you find a lot of old pieces done one way (for whatever reason) and it seems to have held up, then I'd be inclined to do it that way myself
For the most part it’s also in the grain and cut of the board(s) being used. Less so for 1/4 sawn vs others. Projects are always full of surprises. The thickness of the material also can have a large effect. Finishing both sides doesn’t always work if your moisture content is off (low or high). The wood will most usually seek a balance between the local humidity and the moisture content within.
Experts tend to have ossification-of-the-belief, for the usual human reasons. Sometimes their adamantine view can be cracked a bit by the forces of reality but in other cases they remain statuesque in their attitudes ....
Surely there is somewhere a well-conducted scientific experiment or fifty that examine the process of wood take-up and loss of moisture in various controlled conditions, including some involving differential covering with various oils, varnishes, paints and so forth? Objective (measured) data from experiments made in which other factors are controlled or eliminated tend to produce more resilient expertise about such matters.
Anecdotally .... I know that plain-sawn planks (those showing the C-section of the growth rings) can lose moisture differentially from their two wide sides. But this can occur in many situations, not just where one side is finish-painted and t'other isn't.
I once had a dozen 6 foot long, 6 inch wide and 5/4 thick ash planks that had been air-dried to 15% moisture content (measured with a Wagner moisture meter). I had them in doors to acclimatise to the normally 10-11% moisture content of furniture in centrally heated houses. They were laid one face down on 1 inch thick stickers on a wood floor in a spare room. Every day, whilst drying from that 15 to 10%, they cupped. When I turned them over (every day) they would flatten once more and even cup a bit the other way.
Eventually they stabilised and remained flat (ish). No finish involved there, just what I presume to have been a faster loss of moisture from the side facing up rather than at the floor.
Lataxe
This forum post is now archived. Commenting has been disabled