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Other than an orbital sander being circular and a finish being square, I was wondering if there are any other differences. If I already own an orbital one, does it make sense to add a finish one to the portfolio? I am currently doing some antique doorway and window trim restoration, I chemically removed the paint and now doing final polishes before I stain and lacquer.

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antique doorway and window molding? i presume that means very few flat surfaces, in which case you will ruin the profiles with any electric sander. all your sanding will need to be done by hand. –  mike Nov 4 '13 at 18:30
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1 Answer 1

Anything that sands across the grain on wood puts scratches in the growth rings that can be hard to remove.

Orbital sanders are good for removing homogenous materials like paint where you don't want a pattern to the marks left by the sand paper. As you use finer paper, the random action leaves finer scratches till the surface is smooth.

Finish sanding on wood is a bit different, you want a straight line sander so the abrasive cuts parallel to the wood grain so you can get a smoother surface.

Also, Finish Sanders are available with Random Orbital Action. If you've ever been in a car paint shop, you know it as a DA or Dual Action Sander. The pad is loosely coupled to the orbital mechanism so it doesn't produce spiral cuts in the finish but has a random combination of straight and orbital motion.

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"parallel to the wood grain" -- but the wood grain is highly irregular in shape. –  amphibient Nov 4 '13 at 16:42
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The wood grain may seem irregular, but scratches across the grain are more visible to the eye than scratches that run nearly parallel. At least that's what I've noticed. It's similar to why you plane with the grain and not across it. You get wood tearout going cross grain. –  Fiasco Labs Nov 4 '13 at 16:45
    
so for this application, would you use an orbital or a finish sander? –  amphibient Nov 4 '13 at 16:51
    
@FiascoLabs describes the differences between random orbital and straight line sanders well. This answer, however, may seem to imply that finish sanders are straight line sanders. Some, but not all finish sanders are straight line sanders. There are orbital action sanders sold as finish sanders as well. –  mac Nov 4 '13 at 17:55
    
Make sure it says Random Orbital Action for your finish sander, standard Orbital Action leaves small round spirals. As to the application, I would use the orbital action sander to remove the finish and a finish sander during the last fine sanding operations and inspect the surface for marring scratches. Rub it down with a tack rag to remove the dust and give it a visual inspection. Sometimes the scratches show up only after finish is applied, so choose an inconspicuous test piece to get your technique down. –  Fiasco Labs Nov 4 '13 at 18:15
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