MDF Panel Warpage

      MDF panels may warp after finishing if the back of the panel is not also finished. August 30, 2005

I've seen this asked in many forums before. When finishing any panel type product, it's best to finish it on all surfaces, to prevent movement of the panel. I’ve been a firm believer of that for many years.

Here's the situation. The shop I work for recently finished several large, 3/4" x 72" x 72" MDF, veneered panels. They have a herring-bone pattern on one side, a single row, and veneer on the back, edge banded on four sides. The panels started out very flat. The panels were sealed on all sides, stained on the front and edges and then three coats of conversion varnish on the front and edges.

They warped outward at least two inches in both dimensions, length and width. My thought is that they should have had topcoats applied to the back face of the panel also, since a coat of sealer seems as though it would leave the panel more susceptible to drying out from the backside, thus causing the warping.

Has anyone had problems of this type with large veneered panels and how did you deal with it? We've already stop kerfed the backside of the panels to make them more usable for our application, but would prefer not to have to do this in the future. Any help is appreciated.

Forum Responses
(Finishing Forum)
From contributor L:
Today’s modern finishes shrink a small bit as they dry. If you look at the finish after it flashes you will see some orange peel. As the finish dries it stretches itself out and smoothes down. This shrinking is what is probably causing your warpage. You need to do the same to the backs as you do to the fronts, three coats of CV on both sides. Two coats of (unthinned) CV should be sufficient. Most CV are over 40% solids and doing three coats would put you over the mil thickness limits.

From contributor D:
I have never had 3/4 MDF curl that bad. Plywood, yes, but MDF is usually as stable as a rock. Are they horizontal or vertical, and how are they supported? Are any of them swelling?

From the original questioner:
Contributor L - My mistake. Only two coats of CV are being used. I agree with you on the finish shrinkage. Contributor D - The panels are mounted to a wall using z clips. As far as swelling goes, the thickness of the panels hasn’t changed. They are bowing outward. The main issue now is getting shop management to at least consider the possibility that top coating the back of the panels will prevent this problem from occurring in the future.

From contributor T:
Always apply the same amount of coats to both sides of MDF, plywood or solid lumber. Amount of coats varies on how aggressive sanding is done between coats and how much reducer is added to your finish.

From contributor C:
How many layers of veneer are there? Did you cross-band before face veneer? A 5 ply panel will be more stable.

From contributor M:
To the original questioner: Ditto to those who say equal treatment to all sides. Contributor C, I am going to have to disagree with you on the 5 ply. I understand the theory behind, this, but in reality, MDF had always been more stable for me. There are just too many issues to deal with when it comes to veneer core substrates. There are glue issues, moisture content of the plies, integrity of the plies, and the list goes on. MDF is a much more uniform of a product. Again, I understand the theory, but that same reasoning would suggest that my 15 ply Baltic Birch would be even more stable. Fact is it still moves.

From contributor D:
Was the kerfing on the backside done before or after the warping? If the core is kerfed from the backside my guess is that the problem lies there. The act of finishing the back panel to balance it with the front will be negated by the kerfing, especially if the kerfs were more than 1/8" deep. If you went 3/8 - 1/2 " deep with the kerfs it almost surely will bow toward the unkerfed side.
You may need to find a way to fasten some kind of stiffeners to the back of the panels to pull them back where they belong. Is there room to do that?

From contributor R:
How were these panels veneered - in a heat press using urea formaldehyde glue? If so I don't see how the panel could dry out from the backside, assuming we have checked our moisture content and let the panels cool properly before any handling. If the panels are bowing toward the face that would imply moisture being picked up by the back side thus making it bigger. Did you use the same thickness and similar species of wood to veneer the back? Unbalance there can cause problems also.

From contributor S:
I like to veneer over melamine panels to prevent the problem of warpage. Using two-sided melamine creates a panel, which is pre-sealed and balanced before you even put on the veneers, and adds very little cost. You just have to kill the gloss on the side you are gluing to.

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