"Pre-sanded Wood"

This post will concentrate on the sanding of wood.


HMM...."I don't need a multiple head sander. I bring all my product in pre-sanded."


Head Scratch..."We just touch these up by hand. They all come in pre-sanded."


Call 911..."Our company sands our veneered particle board 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."


Although I am paraphrasing these statements, they are entirely too common. The three examples above are kind of a poor, poorer, and poorest comments I hear almost daily.



I'll explain them all in a little greater detail in a bit, but first I would like to cover a few things that often get overlooked.










Why do we sand anything? What is my goal pertaining to the surfaces of my workpieces?


When I ask the first question, 9 out of 10 people will give me part of the right answer. "Too make the surface smooth."


Correct........kind of.


Actually, we need to sand the surfaces of the wood for another reason, just as important. We can demonstrate this with a simple procedure some of you may have heard of. It is commonly referred to as The Water Drop Test.




Source: Douglas J Gardner "The Relevance of Surface Properties & Wood Finishes to the Wood Science & Technology Research Community“






The above picture 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 above were 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 2 passes of 220 grit sandpaper, you 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 5 passes of 220 grit sandpaper.


The best write-up I've seen written is published in the book "The Encyclopedia of Wood" - by the U.S. Department of Agriculture. Chapter 9 is the relevant portion of the book.

In a nutshell, when you sand the surface of the wood, not only are you making the surface smooth, but are also reactivating it. Extractives from within the wood cause a poor wettability 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, but also with time. This is why we have 2 very important thumbrules 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, while some are more acidic. This is also affected by sanding the surface. Whether it be adhesive or stain or polyurethane, sanding the surface allows the bond to be formed not only mechanically but also chemically.



So, the problem with the first statement is that normally the grit I need to use to uncover a raw, active wood surface is not the final scratch pattern I want. If I do use my finishing grit on one head with one pass, I will either prematurely load the belt, or I won't take enough stock off and my surface will be blotchy. Below are three pictures at the cellular level of wood. One is normal, and 2 are shown crushed by using a dull or loaded sanding belt.








How well do you think the wood from that third picture finishes? Not good.

Ok, what does all this mean to my business? One example is easily shown on the water drop picture above. With the daily increase we see in water based stains and coatings, it seems that I would have to only use a fraction of the coating on the activated surface leaving much less to wipe off. Obviously, the less I have to wipe, the more thats in my bucket. That will save a suprising amount of money in a short amount of time. By allowing your stain to wick out evenly, will make for a much more uniform color not only within one part but also part to part. Also, if I don't finish and seal the workpieces I produce, why sand to the ultimate finish grit? Your customers need to sand everything again anyway. Yes, please remove defects and for sure any and all veneer tape, but a 150 grit is usually more than acceptable. You instantly cut your abrasive budget in half. That's huge and that is the problem with the 3rd statement.

Nothing can sand as delicately as my hand, but it's pressure on the board is all over the place depending on what I'm thinking about.....vacation, speeding ticket, etc. The machine gives you consistency, which is the problem with the 2nd statement. Every part may look great but that grain matched display case looks nothing like the desk, which in turn looks nothing like the cabinet doors even though they were made from wood from the same tree.

Ok, ok.....I'll get off my soapbox now.

Thanks for reading, please let me know if there are any comments or questions.

Lastly, I want to give a special thank you to Buetfering GmbH for supplying the photos in this post.