What Causes Raised Grain on Sanded Wood?

Posted by Sandpaper America on Mar 25th 2019

What Causes Raised Grain on Sanded Wood?

For many woodworkers, raised grain is a common issue. After sanding your wood down to a smooth, flat surface, a day later the flat surface is no longer flat. There are actually a few reasons why this happens, all of which can be prevented with some simple changes to your sanding routine. Continue reading to learn what causes the grain to raise on wood that has just been sanded, and how to fix the problem.

Wood Grain Surfaces Changes

Often times, you don’t notice that your wood has warped until you apply a glossy finish. The reflectiveness emphasizes the dips and bows on your work piece. So why does this happen? There are two primary reasons why wood experiences surfaces changes after it has been sanded smooth.

#1: Uneven Moisture

Wood contains an uneven amount of moisture, and this moisture stays within the wood even after sanding. Once it has been sanded, the moisture levels change as a result of shrinking and swelling. And this contributes to the surface alteration of the wood. These changes can range from barely noticeable or minor, to major or very extreme. Warping, distortion, bends, bows, dips, and more are common grain surface changes seen as a result of and even moisture levels in wood.

Managing Wood Moisture

The remedy for post-sanding wood grain surface changes is not so simple. The only way to prevent and uneven moisture from affecting your woodworking piece is to ensure that the moisture within your wood is uniform and even before manufacturing, and then apply the necessary methods to keep the moisture controlled within your woodworking piece to prevent it from experiencing significant surface changes.

#2: Compressed Wood Cells

The second cause is much more complex to understand. Essentially, it has to do with the structure of wood and its growth seasons. You see, wood retains tiny cells within its fibers. In the early springtime as these fibers grow the cell walls are very thin, so they are weaker, making the wood softer. As the season progresses, the cell walls thicken and become stronger, making the wood harder. For instance, if you cut into a piece of Southern pine you would see distinct rings in the wood that represent lighter-colored, softer, “earlywood” and darker colored, harder “latewood.”

So how does this relate to sanding and service changes? The answer is also complicated. Basically, whenever a knife, sawtooth, or abrasive is applied to a piece of wood with uneven moisture, the force created can compress the weaker “earlywood” cells. The blade or abrasive can’t decide if it is more effective to cut through the fiber or simply push it down and out of the way. And this is what causes cell compression.

Common contributors include sawing, veneering, planing, and sanding (especially if the sandpaper is dull). Most of the soft cells will experience something called “spring back”, meaning they return closely back to their original size. But that some cells will remain permanently compressed, so the combination creates the uneven surfaces in wood.

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