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I am relatively new to woodworking. I am in the process of laminating plywood (which will be used as a tabletop and also to hide the actual support for the table). I have googled alot and found different answers. Some are not even for plywood.My question is do I need to laminate both sides?

The cost and time will be high if I do laminate.. Some parts of the plywood will be sealed to the wall so that will be useless to laminate. Can I get away with laminating one side and just coating the other side with varnish or something else (please recommend me).

The humidity here is almost constant,

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David
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    Will the table be used indoors? If so, you need to look into indoor humidity. This is mainly affected by heating and cooling - so you have lower humidity in winter when heating is used and high humidity in summer when cooling is used. – Eli Iser Nov 24 '16 at 15:14
  • Kudos for the well-asked Question. Unfortunately the main issue here I think is going to be something we can't know, which is the quality of the plywood you're working with. So regrettably some or most of the advice here has to be taken on an it depends basis. So take the following as just a very basic guide: if your plywood is good quality and thick, no you don't need to laminate both sides. If not doing both sides it's better to use contact cement than any glue that contains water. [contd] – Graphus Nov 24 '16 at 19:14
  • If using cheaper and/or thinner plywood and the cost of laminate for the back as well would be prohibitive it is worth painting something on the opposite face, but just to be careful rather than because it's a must-do. I would recommend you use any decent oil-based polyurethane (or other varnish) for this and apply three full-strength coats by brush or roller. Let it dry before laminating the face side. Best of luck! – Graphus Nov 24 '16 at 19:18
  • I am not sure about the grade of plywood but it's 18mm thick. There is no climate changes here where I am living so the room where it will be used in only has a air conditioner for cooling.

    I am just gonna play it safe and laminate both sides (laminate the other side with cheaper veener as @Martin suggested down in the answers.

     – David Nov 25 '16 at 15:56
  • FYI air conditioners do dry the air while cooling it (cold air holds less moisture than warm air). But whether this is a factor here I don't know (don't think so). 18mm plywood is inherently strong and stable so that's a good starting point, 12mm and thinner would be much more prone to warping and even with those experienced users don't always counter-veneer depending on the size of the piece. – Graphus Nov 25 '16 at 16:22

3 Answers3

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Part of the answer depends upon the size of the piece, its thickness, and possibly the number of layers in the panel. I do not know any hard and fast rules for making a determination but here are some considerations.

  • The grain in each layer is set perpendicular so that strength in the panel resists the tendency of adjacent layers to curl as moisture and temperature generate distorting stresses.
  • Plywood panels have odd number of layers so that the opposing forces are balanced helping the panel keep a flat profile.
  • If the plywood panel is small (<18" width and depth) and the panel is thicker (3/4") adding a thin veneer layer on one side only will probably work. As the dimensions increase or the thickness decreases the risk of distortion increases. Of course, if the surface applied on the face is thicker, it will generate more stress in the panel.
  • Painting one side will not prevent moisture penetration and curling. Moisture is tenacious and will find a way in generating stress in the panel. The edges of the panel will be especially vulnerable to admitting moisture. However, the steady humidity in your chart is a good thing and will help.
  • There are other panel materials such as MDF or particle boards which can be used as a substrate that are not subject to the same stresses that could be used. For instance, kitchen countertops are only laminated on one side (with a waterproof membrane no less) and maintain a flat profile for long periods. despite humid conditions.

If there is a design reason such as maintaining a specified thickness for only veneering one side, then it is worth a shot, otherwise I would recommend surfacing both sides.

Ashlar
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    Excellent Answer! On your point #4, the right paint or clear film finish will greatly slow moisture transfer on the large flat surfaces. While this is a potential benefit on solid wood it's not really an issue with ply as any tendency towards warping is largely from water (if any) in the glue, not from atmospheric moisture. – Graphus Nov 24 '16 at 19:07
  • @Graphus. Well that is a surprise, I learn something everyday. How does the water in the glue generate warping stresses? – Ashlar Nov 26 '16 at 03:33
  • The usual way water generates warping stresses :-) Basically the same principle as can cause a freshly milled board to warp if laid on the workbench overnight before use — the upper face can dry out while the lower face is protected from moisture loss. When you wet one side only if it's constrained from expanding by cross-grain veneer or plastic laminate you can get compression set. Also glue-shrinkage effects are possible depending on the glue (hide glue being very strong) so it's quite complex what can happen. All together I know I'd much prefer to do anything like this on ply or MDF! – Graphus Nov 26 '16 at 08:46
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As others have said, "it depends". On the other hand, plywood is much more stable than solid wood, so a thin veneer on the top only will probably be OK.

Please note that it is not necessary to use the same wood for the top and bottom veneer. Traditional veneered work used expensive high quality veneers (for example, walnut burr), for the outside and a much cheaper "backing veneer" on the inside. That kept the forces balanced, without having to waste money on expensive veneers that wouldn't be seen.

Martin Bonner supports Monica
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As a rule of thumb, when laminating or veneering any panel, you want to keep it balanced, meaning applying the same, or close to equal finishes to each side.

For one sided, good one side panels, we will apply a backer to the unfinished backside to maintain a balanced panel. If you have laminate on the face, you ideally want a laminate on the backside. The liner can be a commodity product, and doesn't need to match the color of the face, so it can be something less expensive.

Plywood is a core that is far more likely to warp than other options like MDF or even particle board. Laminating a single face will certainly exacerbate the warping, depending on the size and application.

Here is a really great resource from Formica on the subject.

Formica® Laminate by formica Group / Avoid Laminate-Clad Panel Warpage Technical Brief

Volfram K
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Jacob Edmond
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