When I'm sanding I have a 20" box fan setup on my bench with 2 filters. I
have the good 3m filter (forget how small a micron it filters but it gets
pretty fine). then I have a cheap fiberglass filter outside of it. I made
like a J-channel for them to sit in then tape the edges to keep it sealed.
Does a pretty good job.

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Mike S.

http://members.tripod.com/n0yii/woodworking.htm