If I asked you “What is the most important thing about sanding?” what would your response be? Maybe the grit, the type of backing the sandpaper is made of, how fast you sand, or the pressure you use while sanding…each of these, along with several other considerations, is very important.
However, I would like to shift your focus a bit and ask you to try to look at it from a different angle. The scratch.
In essence, what are you doing when you sand any type of material? You are installing a series of scratches on the surface of that material to flatten it out to a point where it looks pleasing to the eye, feels smooth to the touch, and accepts an even coat of finish.
When choosing your beginning grit, you want to start as fine as possible and still be able to flatten any imperfections in the wood in preparation for the following grit. Beginning with as fine a grit as possible will save you time sanding (everyone’s favorite part of woodworking — sarcasm intended) and save money in sandpaper.
Take plywood, for example. When sanding cabinet-grade plywood, you don’t want to start with 80 grit as the surface has already been prepared to around the 120 grit level; I typically start with 120 or 150 grit, depending on the specific application. However, if you begin at 80 or 100 grit, what you are doing is going backward in the process and thus running the risk of “burning through” the thin veneer that serves as the finished surface of the plywood.
Avoiding pigtails, swirls and bad scratches: Case study
The speed at which you move your sander across the surface of the workpiece also has a direct effect. The backing pad of a random orbital sander spins in an orbit or a circle. At the same time, there is an offset weight in the gearing of the sander that is designed to throw the entire backing pad off-center as it orbits. This dual action (this is where the term “DA sander” comes from) creates what is known as a “pigtail” scratch. However, if care is not taken during the process, these pigtails can become your enemy, rearing their ugly heads only after the stain has been applied.
One recommendation is that you should move your sander across the workpiece at a rate of 1 inch per second. A faster speed will elongate the scratch pattern and give you a different finish.
I recently visited a large cabinet company that was having a few issues coming out of its sanding department. The first was swirl marks that could not be seen until stain was applied. The second was inconsistent color at the same stage.
I had the occasion to watch the sanding department in action and focused my attention on two of the sanding personnel who were working side-by-side. They were both sanding cabinet doors of the same wood species that, I assume, were destined for the same finish. But their sanding techniques were entirely different. The more experienced of the two moved the sander very quickly from one end of the stile to the other several times, stopped to focus on the rail/stile joint for several seconds, and then moved on to the rail. This pattern continued around the entire frame of the door, at which point the random orbital sander was abandoned for a foam-sanding pad, which was used to lightly sand the flat panel in the middle of the door. Note: The center panels are thoroughly sanded before the door is assembled and this light sanding is just insurance against any light scratches the panel might have picked up since assembly.
The second member of the sanding team was less experienced and as it turns out, was very new on the job. As this person had just recently been trained on how to sand the door, she was moving the sander at a much slower pace, allowing the random orbital motion to perform the action that it was designed to perform. This person did not have to work as hard or as fast but was leaving behind a more refined and even scratch pattern.
The point is that these two cabinet doors had two very different scratch patterns on the surface, increasing the likelihood that they would take the finish differently and upon inspection, would not look exactly the same coming out of the finishing room.
In order to avoid issues coming out of the sanding department, have your sanding staff ask themselves, “What kind of scratch am I leaving behind?” A consistent scratch across every workpiece will help to ensure a consistent finish.
The first sander left an elongated pigtail scratch because she moved the sander very fast across the surface of the piece. The second sander had a much tighter and more even scratch pattern that is more conducive to an even finish coat because of a slower sanding speed. If the goal is to have a consistent finish from one piece to the other, the sanding techniques of all sanding personnel should be the same.
The second issue I noticed from watching both sanders is that they would sand the length of the stile, stop, and then reverse direction and sand the stile again. Then they would stop at the other end, and repeat the process. This technique leaves a more intense scratch pattern in the corners of the door than on the center sections of the rails and stiles.
A more even scratch pattern can be obtained by engaging the sander on one corner of the door and slowly moving it around the frame of the door three to four times. Then lift the sander off the piece before disengaging the motor.
Now you might say, “You’re just nit-picking,” and maybe I am. But if the sander’s job is to leave behind an even, consistent scratch pattern, we have to examine techniques from sander to sander to ensure that we are getting the same product from everyone in the sanding department. Once we are confident that we are receiving a consistent product from every member of the sanding staff, we are much less likely to have to re-sand a stained piece because of inconsistent finishes.
I would recommend the following: Create a sanding spec. A list of do’s and don’ts that focus your sanding team on a specified technique. Teach it to the team lead and then to the individual team members. Print off some small signs that remind them of the scratch they are leaving on the workpiece and post them at each workstation.
What is the most important focus when sanding? Is it the grit, the sandpaper, the brand of sander or the sandpaper, or the speed of sanding? I believe it is the scratch.
Tips for consistent sanding
- Move the sander at a slow speed. Let the random orbital sander do the work it was designed to do. The recommended speed is 1 inch per second. You can move a little faster than this but the point is to be consistent across the entire surface of the workpiece.
- Avoid excess pressure on the sander. The weight of the sander itself and your hand is sufficient to smooth the surface and provide a good scratch pattern.
- Avoid tipping the sander to concentrate on a specific area. When the sander is tipped, it causes the abrasive grains to dig deeper in that area and creates a scratch inconsistent with the rest of the piece.
- Start the sander on the workpiece and lift it off before disengaging the motor.
- Blow the workpiece off between grit changes.
- When sanding, try to maintain continual motion. Stopping your sander’s forward motion on the workpiece creates a more intense scratch pattern in that spot.
- Never skip more than one grit at a time in the sanding sequence.
Source: Nick DeMars is a product specialist at Klingspor Abrasives Inc. For information call 800-645-5555 or visit klingspor.com.
Have something to say? Share your thoughts with us in the comments below.