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Finishing Both Sides - Hoadley vs. Flexner3/7
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. Iíve never had an issue with warping or cupping when using an Oil finish, however I consistently experience these issues when using a Polyurethane.
I don't ever want to temp fate. I finish both sides even if it's just belt and suspenders kind of insurance. Now some finishes used in a closed cabinet can stink forever. Oil is one of those. I've used shellac or water based poly on the inside and then whatever the spec is on the exterior.
Personal experience with a rather expensive quarter sawn white oak countertop backs the standard thinking of balanced panels. Many saw kerfs filled with epoxy was required to make it flat.
There is a reason why you are supposed to put backer sheet on laminate countertops.
Balanced panels is like gravity...not climate change.
Did Bob recently have a head injury? Did he start huffing his shellac solvent brew?
IIRC Hoadley points out that oil finishes have little to no impact on atmospheric
Bob seems like a nice guy, but he has no meaningful qualifications other than "i've been doing it a long time". He's been running a shop for 30+ years.
Which is totally great. Experience is a wonderful thing.
Meanwhile, Bruce was a professor of building materials and wood technology, who did many years of actual research (and literally wrote the book) on how wood is dimensionally affected by moisture.
Oh, he also was responsible for helping develop a significant number of adhesives and finishes, too :)
Bob takes strong issue with Hoadley's viewpoint, but he never even seems to have taken the basic tact of "asking hoadley"
All based on like three sentences hoadley wrote back in 1980 :).
He also simply assumes he understands all science related to this.
But of course, the person who actually does here is Hoadley.
The actual cause is the unevenness in drying rate, *not* the moisture content change itself.
He then says if they are both sides finished and you change the humidity "(warping) will just happen slower", but this is simply wrong.
As Bob himself points out, if they are finished unevenly, the moisture will escape unevenly - which means warpage, since one side is drying faster than the other.
If both sides release moisture at the exact same rate, it will not cup.
He claims this all mainly is driven by "inadequate kiln drying" which is .. beyond silly. It is true that some wood is not dried properly, but kiln dried wood is actually *too* dry for most homes.
Kiln dried wood is 6-8% moisture content.
The real driver is usually that relative humidity in most homes changes on a regular seasonal basis due to heating and cooling.
In fact, in most homes, it's very sudden, going from "no heat" to "heat".
Just the kind of sudden moisture content change that might, you know, warp a board.
But you don't have to take my word for it, because Bob could simply read all the research that covers this in great detail - written by Hoadley :)
It's very easy to convince yourself Bob is huffing something here. The concept he seems to miss is uneven drying rate and permeance.
The fun thing about science is that it is testable.
Take two boards, same size/thickness/etc. Cut from same part of same tree, have same grain pattern.
On one board, coat everything (including edges), except one face, with a low permeance coating (redgard or something)
On another, coat everything.
Suspend both boards in an environment with seasonal RH changes.
Watch what happens over time.
I don't have time to hang a couple boards and wait to see what happens
What does happen over time?
Sorry, though it was obvious from the first sentence ("Bob is huffing something"), i'll be more clear next time :)
The one that is not coated on all sides will warp.
Usually pretty quickly, too (IE a year or less).
The one coated on all sides will not warp in this manner.
In practice, I'd bet finishing all sides can make the moisture absorption and drying rate slow enough that it's the same year-round in the face of RH change, but i'm too lazy to calculate it.
I rarely coat my countertops equally. Usually it full system coat on the working surface and a seal coat on the back, maybe another coat just to make it smooth. Never a problem.
I try never to leave a raw side.
Overall I think that Hoadley is right; at least based on my personal experience. That being said, I think it does depend to an extent on the type of finish being used.
One of Flexners main arguments was that historical pieces were usually only finished on the outside, and they have survived just fine. Historical finishes were not the most resistant towards water vapour, so the difference in moisture exchange between finished and unfinished surfaces was negligible. Modern finishes are far more resistant towards moisture vapour, making the difference in exchange between a finished and unfinished surface much more substantial and unbalanced.
I suppose it could also depends on where you live. If you live in a fairly stable climate that doesn't swing much in humidity, you're less likely to see an issue. If you live in an area that has significant swings in environmental conditions, you're far more likely to see the impact.
One of several examples I've experienced was a 26" wide panel of Jatoba. It was glued up from three boards and sat in my shop for about 4 weeks unfinished. In that time it did not warp or cup at all. I coated a surface in Linseed Oil and left it for two days, and it was still flat. I then brushed on a coat of Polyurethane. I checked in the next morning (less than 12 hours) and found it had cupped quite significantly. Environmental conditions inside and outside the shop were unchanged. The Linseed oil did not resist the moisture vapour enough to cause a problem, but the Poly did.
I once installed a 3' wide peninsula formica top ... formica on the top side, raw/unfinished particle board on the underside. It was sumertime
Particle board comes from the factory around 6% MC. Within a week or two, the cabinet guy had a call back, I went out, and when I laid a straight edge across the
A sacrificial layer of formica on the bottom side woulda prevented the issue.
Treat both sides equally - it's a no brainer,
And at very least, what Leo said:
Like I wrote in my first post. Formica backer. That thin cheap brown nasty sharp stuff that keeps the panel from cupping.
If you want to see what happens quickly take a test piece of wood or a piece of ply 8-12Ē wide(across the grain). Slather a sloppy coat of film finish(shellac, poly, epoxy, whatever. On 1 side. Let it cure overnight. The next day if it hasnít cupped/warped, put it in a bathroom in your house where the shower is used each day. Check on it the following day.
In one of the other forums this week someone was asking about 1-side UV prefinshed maple cabinet ply. Basically, why does it warp so badly? What can I do about it?
Just a guess. Bob would see things like furniture and tables coming into his shop. They are still looking pretty flat after a hundred years. The difference is in the construction. Tabletops are fastened hard to a frame. The vast majority of furniture panels are fixed or held in place. Some of the finishes(linseed oil?) soak far enough into the pieces to give some protection to each side???
I absolutely think about it when we are building things. In the past we did tons of prime/seal only work. It was all finished onsite. We would brush or spray all exposed surfaces with some universal primer or sealer. If we are finishing a tabletop we will either do 1 heavy soak sealer coat or 2 normal coats on the back side. Depends on gravity or absorption of the substrate.
In the shop there are two ways to deal with fast moisture penetration of unfinished wood panels. Expose both sides to the air or physically restrain them until you can get a vapor barrier on both sides.
If you wanted to make (10) 3/4Ē x 10Ē x 20Ē cherry raised panels. We ignore the cut & flip bs because with cherry itís all about the face grain looking good and the trees are small. We glue up & mill to flat panels. We might not have time to raise them. We leave them overnight.
If we stack them thoughtlessly on a work bench inevitably moisture will find itís way into the exposed side of the panels. By morning several of them will have cupped. Potentially enough they will not feed properly thru the shaper to raise. If we are experienced we will throw some sticks under the panels to allow the moisture to enter from both sides. The other option is to stack them and clamp them to the work bench until you are ready to raise them.
Once again itís real. Stop huffing the product.
Next time you happen to be in HD or Lowes, take a look at the post formed countertop section, where they store them standing up. It looks like a rack of bananas. Formica on only the tops.
I recall being a part of the One side vs 2 side thread that Bob was participant on as well. I think you all are missing what he was trying to point out a little bit. In context, a few guys were debating whether it was necessary to coat the unseen parts of their cabinet boxes/ built in furniture ect... At this time some guys were saying that they were black lacquering the outsides of their boxes and things like that, and I think he was pointing out that this is most of the time unnecessary for in home furnishings. I dont think he was speaking in regards to solid wood table tops and the like.
Wood floors are not coated on both sides?
"Wood floors are not coated on both sides?"
This example helps prove the point
The non-two side coating is done for cost, not because "it works".
This is very explicitly why
2. Why they have to be acclimated to the room for a few weeks minimum to avoid warping (and warp anyway depending on moisture changes)
3. Why the solid wood flooring warps anyway ;)
Have you ever installed flooring? Itís nailed down 1000ís of times in small areas.
Speaking from personal experience.
I had a water pipe burst on the 2nd floor of a building.
The first floor had 1x6 SYPine. Typical poly topcoat. Itís a floor thatís 32í x 32í. One post in the middle and a short load bearing wall. 12-16í lengths. It had water pouring over it non stop for a week. I saw pictures of it with 1Ē of standing water.
The floor buckled in two places across 32í of width. 2 boards kicked up at 45 degrees. The rest of the floor stayed flat.
Lots of nails or screws will restrict movement to an amazing amount.