Wood as a Substrate

Wood is a dynamic substrate. The selection of a species and more importantly, how it is prepared prior to and during the finishing process, has a major impact on the final product. Unlike metal, various factors, characteristic of wood in general, can and does affect the final outcome of the finishing procedure. Only if these factors are taken into account during the finishing process, can acceptable results be achieved.

 Color:  Most finishing operations utilize stains to change the color of the wood. Stains generally enhance grain definition of the wood as well as highlighting the character of the substrate. They are essentially transparent. With this in mind, it is important to remember that the color of the wood will have an impact on the color after staining. For example, staining a piece of white oak (greenish-yellow) versus a piece of red oak (reddish-brown) with the same stain will result in two different end colors. This can and often does occur on even the same species of wood. There are ways to minimize color differences of the wood but must be taken into account as added steps in the finishing operation. As much as everyone has tried, there is no such thing as a magical stain that will provide a consistent color on substrates which vary in color.

Moisture Content:  As mentioned above, wood is dynamic in that it “moves”. It will absorb or release moisture until it reaches an “equilibrium” with ambient conditions. Ideally, moisture content should be around 6-8%. When wood absorbs moisture, it swells. It shrinks when it loses moisture. Generally, moisture content isn’t looked at until a problem occurs. In a perfect situation, moisture is controlled in a finishing plant but this is often the case.

Excessive moisture in wood can cause problems such as poor stain acceptance, yielding a mottled or blotchy appearance. It can also result in poor adhesion of the clear coats. Very dry wood can also result in poor stain appearance (dark) as well as clear coats which soak into the wood too much, resulting in poor film build.

Temperature:  Temperature is very important not only with the paint but the wood as well. As relatively thin coats of material are applied, the ambient temperature of the wood can be rapidly transferred to the coating. Obviously, cold substrates may cool the stain/coating rapidly and impede penetration of stains or cause flow issues with the clear coats. Cool coatings may slow dry which may cause poor sanding characteristics which may result in an overall unacceptable finish. At the other extreme, the wood being too hot, stains may dry too rapidly, resulting in mottled appearance or in the case of wiping stains, may not wipe at all. Clear coats can heat up, thusly lowering the viscosity which can reflect too much flow, i.e. sags and puddling.

In an ideal world, the board surface temperature (BST) should be between 65-75°F.

Wood Density:  The density of the wood can have a profound effect on how stains/coatings will perform on the substrate. The problem is that the density can change even on the same piece of wood. Stains may “over penetrate” on porous wood or puddle on dense wood. There are some tricks to minimize porosity differences but they are time consuming and often labor intensive. Wood selection is the easiest way to minimize it but often times is not practical.

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