By Tim Middleton
There are three primary concepts in the industry concerning the sanding of wood products that need to be evaluated.
Two of these concern end users of wood-based products and the third concerns veneer and board manufacturers:
#1. “I don’t need a multiple head sander. I bring my entire product line in pre-sanded.”
#2. “We just touch these up by hand. They all come in pre-sanded.”
#3. “Our company sands our veneered particleboard sheets to 220 grit before we ship them. Our customers pay a premium for the convenience of not sanding them again. The parts they make from my sheets are ready to go straight to finishing.”
These paraphrased statements are very common. Unfortunately, they are not completely accurate and should be reviewed. To better understand why these mindsets are faulty, I will first examine some fundamental points of working with wood.
Why do we sand anything? What is my goal for the surfaces of my workpieces? When asked the first question, nine out of ten people will answer with a partially correct response: “To make the surface smooth.”
Actually, wood surfaces are sanded for another reason that is just as important. It can be demonstrated with a simple procedure commonly referred to as the water drop test.
The test has three drops of water placed on the surface at the same time. The drops were then photographed after 30 seconds. The drop on the left has kept its large contact angle with the unsanded wood surface. The surface under the middle drop was sanded with two passes of 220-grit sandpaper. One can see that the wettability is much improved. The last drop has almost completely soaked into the surface. The wettability is even more improved on this surface due to five passes of 220-grit sandpaper.
For more information on wettability, read “The Encyclopedia of Wood” by the U.S. Department of Agriculture.
Chapter nine is the relevant portion of the book. In essence, it says that when sanding the surface of the wood, not only is the surface becoming smoother, but it is also being reactivated.
“Extractives” from within the wood cause a poor wettability, as seen by the left hand drop. These extractives mainly come to the surface of the wood from either heat, such as when the part is in a press, or over time. This is why there are two very important rules of thumb in the sanding world. The first is that all wood should be sanded no greater than 24 hours before it is finished and sealed. The second is that when sanding veneer, it should be the goal of the operator to remove about 1/2 the thickness of the veneer.
Some woods are more alkaline and others acidic. This is also affected by sanding the surface. Whether it is an adhesive, stain or polyurethane coating, sanding the surface allows a bond to be formed not only mechanically, but also chemically.
|These photos show the cellular level of wood. One is normal and two are crushed by
trying to take too much stock off at a certain speed or using a dull belt.
Lets go back to the first mindset: “I don’t need a multiple head sander. I bring my entire product line in pre-sanded.” The issue with this statement is that under most circumstances, the grit needed to uncover a raw, active wood surface is not the final scratch pattern desired.
If one finishing grit is used on one head with one pass, one of three things will happen: the wrong type of head will be used for stock removal, leaving an uneven surface which will be very difficult to topcoat correctly; the wrong type of head will be used for finishing, leaving chatter marks on the surface of the part from a calibration drum; there is a risk of either prematurely loading the abrasive belt or removing insufficient stock and the surface will be blotchy.
Above are three pictures at the cellular level of wood. One is normal and two are shown crushed by either trying to take too much stock off at a certain speed or using a dull or loaded sanding belt. Whether this is done in-house or at a supplier, the end result is the same — a very poor finish.
In examining these images, it is clear that the damaged wood will not absorb finishes correctly.
So, how does all this impact business? Properly sanded surfaces result in beautiful finished products and a business will save time and money by reducing the amount of finish needed to apply to the surfaces. For example, with the daily increase seen in water-based stains and coatings, one would have to use only a fraction of the coating or stain on the activated surface, leaving much less to wipe off. By allowing the stain to wick out evenly, a much more uniform color will result on not just one, but all of the parts.
Also, for veneer and board manufacturers and the third mindset, there is no need to sand workpieces to the ultimate finish grit if the pieces are not being finished and sealed right away. Customers will need to sand them again anyway. Defects still need to be removed, of course, as well as all veneer tape, dents and scratches, but a 150-grit is usually more than acceptable for this. All of which could quickly cut the abrasive budget in half.
As for the second mindset involving hand sanding, the primary consideration is uniformity. Although nothing can sand as delicately as the hand, the pressure applied to the board is notoriously uneven and can be impacted by emotions or time constraints. And while each individual part that is hand sanded may look great, the assembled pieces may not match. A grain-matched display case may look nothing like the accompanying desk, which in turn looks nothing like the cabinet doors even though they were made from wood from the same tree. A sanding machine gives consistency to the product.
Understanding how sanding wood impacts end results can be the difference between producing a satisfactory product and creating a work of art. Don’t let old mindsets get in the way of creating the best products possible for customers.
Tim Middleton is Product Manager - Sanding Division at Stiles Machinery Inc. For more information, visit stilesmachinery.com, call (616) 698-7500 or email [email protected]
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