When sanding before applying a stain, I understand that it is important to sand parallel to the grain, because sanding across the grain will make scratches that will stand out. Is it also important to sand only in the direction of the grain and not against it (with a back and forth motion)?
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Does the Sand differently paragraph here answer your Question?– GraphusJan 30 at 20:14
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Important for what?– CalebJan 30 at 22:39
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@Graphus Thanks, that is helpful. I have updated my question to try to be more clear.– bwrogaJan 31 at 0:35
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If you were to only sand "with" the grain and not "against" it, how do you determine which way the grain goes? Parallel vs perpendicular is pretty obvious, but it's not like rubbing a cat's fur backwards where it's generally easy to tell the difference between head and tail.– FreeManJan 31 at 16:07
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@FreeMan I found an explanation here– bwrogaJan 31 at 16:36
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1 Answer
Is it also important to sand only in the direction of the grain and not against it (with a back and forth motion)?
Usually no. It is rare to need to sand only in one direction, either left or right, or away only and not back toward you, in order to get the desired result.
But because grain is not 2-dimensional — it isn't running only along the length of a board but is very much 3-dimensional, also rising or falling (or both, as commonly occurs around knots) — it is sometimes beneficial to sand either the whole board or parts of it going only in one direction. This is for the same reason that it is common practice to plane wood in a specific direction (particularly in machine planing and jointing) to lessen or prevent tearout.
Obviously the effect is much less pronounced in sanding, where it's only small fibres which can be lifted by the abrasive.
As you might predict, this is lessened the higher the grit used (in somewhat the same way that taking finer shavings yields a better surface). But you can't rely on this when prepping for stain because you need to stop sanding at a lower grit than you might otherwise prefer to — e.g. 150 instead of 220 — in order for the stain to work most effectively (give the best colour).
