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I have a 4' x 4' piece of plywood artwork (acrylic painting) that has become warped. It used to be mounted to drywall using velcro strips, but now the outward force of the warp is strong enough that "peels" off and cannot stay on the wall.

I've tried undoing the warp by wetting the concave side and flattening with weights, which only seems to work temporarily — the warp returns after a few days.

Is there a way to permanently undo the warp, so that the velcro mounting system is viable? Or is there another way to mount this that would be non-destructive to the art and reasonably portable?

One answer in Is there a way to remove the warp in plywood recommends nailing or screwing the plywood to something, but that sounds destructive and non-portable. (Another suggests leaving the plywood in the sun, but sadly I don't have a place to do that near my apartment.)

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How thick is the plywood? Are the edges finished in any way? The plywood will need to have the tension released and that depends on how thick the plywood is... How many layers that is in the plywood is a help too. –  Jack Feb 15 at 20:37
    
1/4" thick, no edge finish, looks to be 3 layers although it's difficult to tell precisely as the artwork bleeds over the sides. –  RKBWS47 Feb 16 at 16:05
    
My suggestion was going to be to score the back of the plywood MANY times with a low set table saw blade to release the tension, then laminate it to another stable backer like 3/8" or 1/2" MDF with a small hole already drilled in it for hanging on a wall by a angled nail or screw. Bond it to the MDF with a yellow wood glue, strategically placed tape to keep it properly located, and everything placed on a flat surface, with another flat panel over that and a lot of weight to hold pressure on the glue till it dries. 1/4" plywood will take only the shallowest of cuts to relief the tension. –  Jack Feb 17 at 0:24
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3 Answers

up vote 2 down vote accepted

Plywood is a lamination of several layers of wood set with the grain rotating with each layer. This actually helps resist warping, but does not eliminate it (as you can see). This happens because of some flexibility in the glue that allows a gradual creep.

The usual solution is as you suspect - laminate the plywood to something else. This could be a metal bar, either aluminum or steel. Perhaps the most effective would be angle stock (metal forming an "L" in cross section). A flat edge of the angle is screwed to the back. The perpendicular edge then acts as a brace.

If you do this, you need to first flatten the piece as much as possible. I would recommend placing it face down on a flat surface, possibly with a sheet below it to protect the art surface. Then weigh down the corners with very heavy weights, such as full paint cans or barbell discs. Lay an angled bar across the full width near the top and another across the bottom. Place screws every 6 inches. The screw length should be long enough to go through the angle stock and 3/4 of the way through the plywood.

You can then rehang the piece using a wire threaded through holes at the ends of the angle stock. If you really need to use velcro, you could attach wooden filler strips to the upright edge of the angle stock, and attach the velcro to the back of these strips.

An alternative bracing can .be done with strips of plywood, al teast 3 inches wide and 3/4 inches thick. These can be glued and screwed to the flattened panel instead of the angle stock. If you use a strong wood glue, such as Titebond Ultimate, this will help prevent creep, as will the screws.

The advantage the angle stock method is that it is reversible (no glue).

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Thanks for the thorough answer. I'm trying to visualize the angle stock solution but not sure I'm fully understanding it. From your description it sounds like the angle stock runs parallel to the plane of warpage, i.e. the horizon. How does this stop the warp action? I sketched my understanding of the construction, do I have it wrong? i.imgur.com/OwkcVjs.jpg –  RKBWS47 Feb 15 at 21:52
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Your right, I assumed the warpage was along the horizontal (not sure why). If it is only warping in the vertical dimension, run the angle stock vertically. If it is in both directions (pretty rare) you could run four bars just in from each edge. –  bib Feb 15 at 22:05
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Maybe you could make a frame for the picture and screw it on from behind. It might pull the plywood back to shape. Depends on how thick the ply is.

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I've tried undoing the warp by wetting the concave side and flattening with weights, which only seems to work temporarily — the warp returns after a few days.

If you paint the back (acrylic paints on the front, use acrylic gesso on the back) when you have gotten it flat, it's much more likely to stay flat - with one surface only painted, it will always warp.

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