Wood surfaces are not only sanded to make the surface smooth! Although that is the most important reason, there is another factor to take into consideration: wettability.
This concept is easily demonstrated with The Water Drop Test. The photo to the right shows the differences in wettability on a piece of yellow birch. The two arrows are pointing to the inactive and activated surfaces.
The test has three drops of water placed on the surface at the same time. The drops were then photographed after 30 seconds. The water on the left was dropped on an unsanded wood surface and as you can see, it has kept its large contact angle with the wood surface. The surface under the middle drop was sanded with two passes of 220 grit sandpaper and you can easily see that the wettability is much improved. The last drop has almost completely soaked into the surface. Why? This section of the wood was treated to five passes of the 220 grit sandpaper.
So, why is this important? Because it reminds us that when sanding the surface of wood you are not only making the surface smoother. You are also reactivating the wood. Both time and heat cause extractives like tannins, oils & resins from within the wood to rise to the surface. Think about how this may affect your finish on the wood. Bottom line: All wood should be sanded no greater than 24 hours before it is finished and sealed.
Properly sanded surfaces result in beautiful finished products. You will save time and money by reducing the amount of finish needed--one would have to use only a fraction of the coating or stain on an activated surface! And even better is that by allowing the stain to wick out evenly you achieve a much more uniform color and finish.
For more information on sanding wood or any other surfaces, please give Abrasive Resource a call at 800-814-7358 or check us out online at AbrasiveResource.com