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Ask the Expert
by Curtis Hicks
Over the years our customers have asked many technical questions about our products, applications, and general sanding advice. Uneeda thought others could benefit from the answers to many of these important questions.
Please feel free to email us at sales@uneeda.com with your questions.
Why do I sometimes get swirl marks or fish eyes when disc sanding with a pneumatic random orbital sander?
Swirl marks, also called fish eyes, are usually a result of insufficient air pressure into the sander causing the sander to slow down. Random Orbital sanders should run with a minimum of 10,000 RPM at no less than 90 PSI of air pressure when the sander is being run. If you run the sander slower than 10,000 RPM's swirl marks may appear. If you drop below 90 PSI at any time during your orbital sanding process, you WILL get swirl marks. In many cases the swirl marks due to insufficient air supply occur infrequently. This occurrence is usually the result of having a compressor too small to handle all shop equipment. In other words, the compressor may produce enough air pressure (90 PSI) when only the orbital sander is being used. However, when a second piece of equipment is turned on that uses air while the orbital is being used, turning on the second machine may cause the air pressure to the sander to drop, thus causing the swirl marks. In this case the compressor does not have the sufficient capacity to handle all the shop equipment.
Although insufficient air supply is the number one cause of swirl marks, the following can contribute to swirl marks as well.
- Starting the sander before contacting the surface and/or Stopping the orbital sander on the surface (remember START ON....STOP OFF)
- Using incorrect sanding abrasives. (dull grains can cause swirl marks. Uneeda's brand of sandpaper, Ekamant, is made with only sharp, premium grains. Most manufacturers use a duller grain to cut costs).
- Improper sanding techniques such as applying too much pressure or moving the sander too fast across the surface. Worn bearings in the orbital sanding machine.
- Lack of sander lubrication. (2 drops of light machine oil for every 8 hours ran. No less).
- Too much sander lubrication. (2 drops of light machine oil for every 8 hours ran. No more).
- Sander was dropped or pad was bent.
- Insufficient dust collection (grinding the dust into the wood can leave swirl marks. If possible all sanding should take place on a down draft table)
- Wrong type of backing pad
- Excessive moisture in the wood.
- Incorrect sander orbit
- Dirty or damaged backing pads
- Wrinkled or torn paper
- Water in the air supply
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