Overcoming sander swirl marks
Why does a random orbit sander leave swirl marks and scratches? January 24, 2001
How can I get rid of the swirl marks and sanding scratches left by my random orbit sander? I am using a 6" porter cable with dust pickup and a 220 grit Mirka pad.
An orbital sander should turn at least 10,000 OPM—maybe yours doesn’t.
The problem could also be your abrasive. I use silicon carbide because the mineral itself is very sharp—no rounded edges, like some of the other, less expensive abrasives. As silicon carbide heats up and breaks down, it shatters, creating more sharp points.
Brian Personett, forum technical advisor
No matter what, the sander you are using will not give a ready-for-finish surface.
Use an abrasive head with changeable and profiled inserts to mount in your CNC router or CNC point-to-point machine.
To sand flat and internal door profile, there is a machine called OMNIA 9O PLUS; a semi-automatic machine that leaves the surface smooth. It can also be used for sealer and topcoat sanding.
Is your pad balanced properly? If you are using a coarse paper prior to sanding with 220, there will be scratches that are too deep for 220 to remove.
I used to use a PSA pad and would get swirl marks up to 220 grit. When I switched to the hook and loop and used the cloth backed paper (like 3M's new Regalite), the problem was solved. Now I can even finish out with 180 and have an excellent finish.
If you’re allowing the sander to pick up speed before putting it on the work, it will gouge the wood. While this may be the rule of thumb on a conventional orbital sander, it is not the case with a random orbit. The sander should be resting on the wood before you turn it on.
Consider putting a contour pad on the sander. They provide better cushion.
Maybe your sander does not have enough RPM. Make sure your paper is not clogging up and creating swirl marks from built up dust or grit.
I work with teak, cherry, mahogany and soft pine and I have a 6" porter cable with adhesive backed paper. I use 120 followed by 220 and no swirl marks.