I've heard from various sources that you must always sand in the direction of the grain. Is this always true? Does it depends on the sanding paper's grit, tool used or the wood being sanded? What are the advantages of sanding along the grain?

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    It just makes it easier to hide the scratches when they're running along the grain of the wood. If you're sanding with 320 grit sandpaper across the grain, the scratches will be so small you won't be able to see them. – dfife Mar 17 '15 at 15:40
  • @dfife you should add this as an answer, instead of just a comment. – guitarthrower Mar 17 '15 at 16:13
up vote 16 down vote accepted

In general yes, you will have a much nicer finish much faster by sanding with the grain.

The larger the grit (60 vs. 100) will leave larger 'gouges' in the wood when sanding. The grit is an 'average' size of the grains and the larger ones can leave deeper marks. Sanding with the grain the marks are much less likely to cut across the wood grain, leaving an obvious trail.

So by sanding along the grain you will need less time with finer and finer sandpaper to get the same finish. While you can do the same thing by sanding across the grain it will take much longer.

An easy example is to take a board that is pretty smooth and in one area sand across the grain, then take a much finer sand paper and sanding with the grain sand the whole board. It becomes pretty obvious quickly the 'damage' done.

Random orbit sanders do a decent job of minimizing the worst of the cross-grain cutting. I've also found that the orbit sanding disks have a smaller variance in their grit.

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    Can you elaborate on "The sandpaper grit tends to be more even" with orbital sanders? It sounds like you're saying the consistency and quality of orbital sander sand paper is higher than regular sandpaper. Is that what you mean? – drs Mar 17 '15 at 16:37
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    @drs yes, that is what I meant. though it's possible I buy higher quality orbit sander discs and cheap sand paper... – bowlturner Mar 17 '15 at 16:38
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    Great answer. Nitpicky, but the grit number refers to particles per square inch. Sure fitting 60 grains in a square inch means you will have larger grains, so kind of semantics here. doityourself.com/stry/… – guitarthrower Mar 17 '15 at 16:55
  • I agree it should be clarified that a lower grit actually has larger particles and leaves larger gouges in the wood. Someone who is not familiar with sanding grits may think that 100 is a "larger" grit than 60. – rob Jun 12 '15 at 17:08
  • Here is a list of grits and comparison. If I have understood correctly the different standards ISO/FEPA, CAMI and a japanese variant also decides on the variance. This means that when you buy a P1500/800 paper you get a mean size of 12.6µm; some pieces are bigger, some smaller. The CAMI standard allows for bigger variance (bad) while the japanese for smaller (good). The ISO is in between. – LosManos Jun 15 '15 at 10:32

Sanding across the grain sometimes cuts faster, but sanding with the grain lines the scratches up with the grain so they blend in better.

Assuming you're going through the usual sequences of increasingly finer grits, it's really most important that the final pass (meaning the finest grit) be with the grain.

In fact, one common suggestion for users of random-orbital sanders is to do a final light pass by hand with the grain to clean up any remaining cross-grain scratches.

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    "...it's really only important that the final pass be with the grain." That's a bit misleading, since at any paper it's not easy to eradicate cross-grain scratches left by the same grit. Obviously the problem is much more pronounced with coarser paper but even with 150 if you sand at 90° it takes some work to remove every trace of the perpendicular scratches if you just stick to 150.. – Graphus Jun 12 '15 at 21:03
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    Clarified. Tnx. – keshlam Jun 12 '15 at 22:37

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