Swirl marks are, unfortunately, inevitable when you sand with a disc sander. The good news is that there are some tips and techniques to make them less noticeable.
1. Use a disc sander with a vacuum attachment. You want to eliminate as much of the stray grit and dust created as possible while you are actually sanding.
2. Use graduated sandpaper grits. For the best finish you should not skip intervals (even though that is a common practice!) Swirl marks left by a 120 grit disc, for example, won't be as easy to remove with a 180 grit paper as with a 150 grit.
3. In between each grit change, blow off your workpiece with an air gun to clean any stray grit off your work.
4. Don't press down on your sander! Let the weight of the sander do the work--simply guide the sander.
5. Finally, your last step is to hand sand with the same grit you last used on your disc sander. Always use some sort of a block for even pressure and lightly sand in the same direction across your work piece.
Before finishing, set your work light at a low angle across the sanded surface. Wipe with denatured alcohol and any remaining swirl marks will be revealed...
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