by David McGibbon, Festool Service Technician
Swirl marks can be caused by...
Little groups of loose grit particles get imbedded onto some areas of the sandpaper. They can then make scratches that show up in the stained finish as darker lines where a denser layer of stain can accumulate. This is common with cheaper grades of sandpaper. Or, where larger grit particles from previous passes with heavier grit papers have left larger loose bits of abrasive.
For the best results, the worked surfaces should be dusted off between changes to finer grit abrasives… and use good quality paper.
The constant movement of the sandpaper across the work piece is critical to good results. A traditional orbital sander should always be in motion in long brisk strokes back and forth with the grain of the wood. That way, abrasions from the small circular orbits get stretched out into long ellipses that line up with the grain. You should use these sanders (RS2, RTS400, DTS400, etc.) like you would use a spray gun… You never want to stop or you will get a buildup of paint. If you stop moving the sander you get a patch of swirl marks.
The pad disk on a Random Orbital sander however does the moving for you. It rotates and stretches the tiny orbits into long curved loops, which, when interlaced, tend to eliminate any sign of swirl marks. The centripetal force resultant from the crankshaft offset causes the pad to spin. This is a weak force and pad rotation can be stopped if too much hand pressure is exerted on the sander. You may even notice that when the sander is hooked up to a strong vacuum, the pad will suck itself down to the work like a suction cup and the spin rate may slow considerably.
Festool dust extractors have a speed control for the vacuum motor that should be set lower when using a Random Orbital sander. The sanding motion with a random orbital sander should not be as brisk as with an orbital sander. Make your passes slow enough to give the rotating pad time enough to erase swirl marks. Use very little extra hand pressure on the sander… just enough to stabilize and control, and keep the pad flat to the work surface at all times.
Another type of blemish that can show up in the finish is where the stain does not absorb into the wood. Stuff getting stuck onto the sandpaper results in rubbing & burnishing instead of cutting. The finish then will not absorb into the wood where the wood fibers are mowed down and impacted instead of cut. The stain is not as dark in those areas and gives the appearance of light streaking, whether it is in a circular pattern or a linear pattern depending on what type of sanding machine is used. The Festool patented "Jet-Stream" pad design provides ventilation for outside air to flow from the center of the pad to the vacuum holes located close to the outer edge of the pad. This greatly lessens buildup of debris on the surface of the abrasive.
When a random orbital pad spins too fast there can be burnishing even if the sandpaper is not contaminated. The rubber pad brake used in Festool random orbital sanders is engineered to hold back the spin rate of the pad. This allows for an optimum pattern of movement where the grit particles on the abrasive actually reverse direction and "pirouette" in small loops as they travel in their circular orbit. It is quite a dance.
Each crystalline grit particle has a chance to cut on all of its facets and the resultant scratch pattern consists of very short strokes that interlace in all directions. After many hours of use, the pad brake rubber will eventually wear and allow the pad to spin faster. When the pad eventually spins too fast, the grit particles will not have a chance to reverse direction and only one half or less of their surfaces will be cutting… or actually plowing.
And at higher speeds there is more of a build up of sawdust between the grits because they don't agitate enough to loosen and shake off the sawdust. And the resultant scratch pattern will consist of longer circular strokes that don't interlace as completely. The rubber pad brake will then need to be replaced to prevent burnishing and streaking. These rubber rings are easy to replace. To obtain a replacement call our service line at (800)554-8741.
If faster surface removal is desired, it is best to use something other than a finish sander. The Festool Rotex is designed for very aggressive work. The Rotex with it's gear coupled rotary-eccentric mode is famous for speedy surface preparation with minimal dust. For the ultimate finish, finalize the surface with the ETS125 or ETS150/3 random orbital finish sander.
How to Choose the Right Festool Sander
Festool Abrasives Brochure