Why Does Your Sander Leave Swirl Marks?


Swirl marks are the bane of a woodworker’s existence. Swirl marks are a common problem associated with woodworking, especially when using an orbital or random-orbital sander. But they are completely avoidable and fixable. So, why does your sander leave swirl marks?

If your sander is leaving swirl marks, it’s either one of these or a combination of a few:

  • Poor technique
  • Sanding pads
  • Air pressure
  • The use of random orbit (RO) sanders itself
  • Sandpaper grit

If you and your sander are guilty of leaving swirl marks behind, stick around. This article will provide solutions on how to minimize swirl marks and how to eliminate swirl marks that have already… left their mark. 

How to Avoid Swirl Marks in Wood

Power sanders come in handy in so many different DIY projects around the house. Swirl marks are a sure sign that something’s been sanded. Any one of the following things could be the cause, but a combination of a few may be the root of the issue.

Poor Technique

A sander is fairly easy to use, but it can be hard to figure out how to use them correctly. A sander is a power tool, and like most machinery, it must be handled firmly and calmly. 

Poor technique is a common problem, especially with orbital, random orbital sanders, and sheet sanders that all incorporate your own hand movement while power sanding your wood. 

The way around this is simple. There should be less of you in the process. Let your sander do its job. Just guide it.

How to Avoid This:

  • Do not apply lots of pressure on your sander. Pushing too hard gets poor results. Minimal pressure without any vibration from the sander is perfect. 
  • Slow down. It’s recommended for RO sanders to move at a slow pace; we’re talking one inch per second. This allows for the machine to do its work and could save you time in the long run. 
  • Check your backup pad. The backup pad on your sander can save your woodworking from markings left by the sander. This pad should be in good shape. If it’s not, you can replace it in a few simple steps.
  • Put the sander on the wood before you start it. Do not start the sander and then put it on the wood, or you’ll get some scratches in addition to your swirls. 

Let the sander do its job. You don’t have to force it or press down on it. Remember, less pressure and speed on the front end will save lots of time that could be spent repairing your swirl marks later. 

Sanding Pads

Sanding pads can be a major cause of swirl marks on your wood. Although sanding pads come in many different shapes, sizes, colors, and even forms, pay attention to which type of disc would be the best fit for your sander and your wood. Check out this article on when to replace sander pads. (Link Here)

As we mentioned, disc backings also have an impact on swirl marks. Backings are made from Velcro, HD paper, cloth, film, or even foam. As a result, the backing material tends to have a lot of variety when it comes to thickness, adhesive bonding, and the ability to withstand heat from random orbit sanding processes. 

How to Avoid This:

  • Do the research. Knowing what type of sanding disc and the backup pad is best for the wood, project, and sander will save you time and effort in the end. 
  • Keep your pads in good shape. When pads get damaged, they may cause swirls. Damaged pads may be caused by the outside edge of the pad curving inward and not touching the surface. This increases the abrasive penetration because only the middle of the pad is touching wood, which causes slower sanding and more swirls. 

It is wise to have a few backup pads in your arsenal if one of them gets damaged. 

Sandpaper Grit

Abrasive quality is important for avoiding swirl marks. Not all sandpapers are the same. What truly matters when picking out sandpaper is what you plan on doing with your wood, whether you want your wood finish to be coarse or fine. 

You’ve seen the grit number marked on sandpaper packaging. Remember, the higher the number, the finer the grit. : 80-grit, 100-grit, and 200-grit. The higher the number means, the finer the grit. 

The general rule in woodworking is to start with a low number (coarser) and work your way up to a higher number (finer) as you get closer to the surface smoothness you want. Check out my video below on sanding for an example of what to look for.

Air Pressure

One of the first things you should do when you find out your sander is not performing consistently and having swirl marks is to check your air compressor and see if it is providing sufficient air pressure if you have a pneumatic sander.

If you are using a compressor, make sure you know how much air pressure your sander needs to work sufficiently. If you do not know, figure it out ASAP if you want to keep sanding. 

How to Avoid This:

  • Limit your usage of multiple tools. Sometimes, you may have multiple tools connected to one air compressor, and this will affect the necessary amount of air pressure for your sander.
  • Unplug everything else. If you want to eliminate the risk, unplug the other tools, and keep your sander plugged into the air compressor. If you need those other tools plugged in, try to figure out which amount of air pressure is perfect for the sander to work. 

How Do You Fix Sanding Swirl Marks?

Seeing swirl marks on a wooden table is the project we’ll use here. Many swirl marks aren’t visible until after stain or varnish has been applied. The swirls can be so fine that they don’t really show until some stain hits them. 

If you have already stained the piece, you’ll have to first address this part by using a stain remover, such as acetone, lacquer thinner, mineral spirits, or sanding. To avoid getting in this situation, test a small patch of your project before you stain the whole piece. If you spot scratches and swirls, you’ll have much less back-tracking to do.

Tools:

  • Random orbital sander
  • Four grits of sandpaper: 80, 120, 150 and 220

Steps:

  1. Clean the tabletop thoroughly. Ensure the surface of your project, in this case, a table, is clean of dust and debris. 
  2. Begin with the 80-grit sandpaper
  3. Put your sander onto the surface of the table. Start it up.
  4. Let the sander do its job. Don’t push on it. Guide it slowly and steadily, with little to no pressure. 
  5. Do not go against the grain. Sand in the direction of the grain.
  6. Repeat the process (starting with cleaning off the sawdust) and go through all the grits.
  7. Check for swirls again. You can use a small amount of stain or apply just a little bit of mineral spirits or lacquer thinner to a small area to tempt those swirl marks to show themselves. 

If you see swirls, you need to do more sanding. Determine if you need to start back at 80-grit or if the swirls are worked out enough that you can begin at 120. If you go slow and steady and work your way through those grits, you should see those marks disappear. 

Final Thoughts

If you find swirl marks on your wood, do not panic. There are many directions online besides the one I have provided myself to help you get rid of those pesky swirls in no time. But, as we all know, the best way to prevent having swirl marks is by not having swirl marks in the first place, so take your time and don’t apply pressure to your sander.

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