Is there any such thing as sanding something *too* fine for finishing?

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I know you can get sandpaper in super-high grits, like 2000 grit and higher. I guess there's a point of diminishing returns for sanding wood projects, but is it possible to use too high a grit prior to applying the first coat of finish? If so, what problems can it cause, and do these problems apply to all types of finish and wood or just certain types or categories?

rob

Posted 2015-03-20T17:45:05.247

Reputation: 15 938

Answers

8

That's a great question. Typically, the purpose of the first round of sanding is to hide surface blemishes in the wood (e.g., machine marks, dents, scratches). The purpose of the subsequent rounds of sanding is to hide the scratch marks from the first round. Usually, you only need to go to about 180 before the scratch marks become invisible.

As you mention, there's diminishing returns, so there's no need to sand even higher. However, if you love sanding, I only see one problem: if you're staining, the stain will not take as well for wood that has been sanded with much higher grits. See the image below (from Popular Woodworking). The top half was sanded to 180 and the bottom half to 600.

Two grits of sanding and effect on stain.

On the other hand, as pointed out by @GlenH7, sometimes "finishing a finish" requires sanding to higher grits if you're looking for a highly reflective finish. But, this is for sanding the finish, not the wood itself.

dfife

Posted 2015-03-20T17:45:05.247

Reputation: 3 159

2This is a good answer, and identical to what I'd just started writing. However, you are quoting (or heavily sourcing, including the picture) from an article on popular woodworking; you must attribute that information in the answer. – Joe – 2015-03-20T18:19:38.307

Good point, @Joe. I'll source it. – dfife – 2015-03-20T18:55:51.440

5

For all practical purposes, I think the answer is "no, nothing is too fine of a grit for finishing." But it also depends upon what you expect the final finish to look like.

If we think about what we're doing with sanding, we can rationalize our way through this process. The surface of wood is an uneven surface with repeating ridges representing the grain.

wood grain

And when we sand, we're knocking down those ridges. The finer the grit of sandpaper we use, the smaller the ridges become.

We have to move to ever finer grits of sandpaper while we're sanding otherwise we're just changing around the ridge pattern on the surface of the material. That becomes obvious when we look at sandpaper under a microscope.

sandpaper under microscope
600 grit sandpaper under high magnification

Sanding is just carving out smaller grooves in the wood, with the depth of the groove being defined by the grit of the sandpaper. Moving to really high grits (like the 2000 you mentioned) merely continues that pattern of decreasing the height between the various ridges on the wood. The net effect is that the material feels more and more smooth.

There is a possibility that using too high of a grit would impede the stain from being absorbed by the underlying grain. Regardless of the stain and pigments potentially being impeded in their absorption, smaller ridges from finer sanding will reflect light differently causing the more finely sanded piece to appear different from the less finely sanded piece.

Keep in mind that when the wood absorbs the stain it will lift the grain, undoing the some of the work that you had put into the finish.

It's also worth noting that some finishes that end up looking like smooth glass require wet sanding with progressive layers of sandpaper all the way up into the 1500 or higher realm.

Finer grained woods are less likely to show as dramatic of a difference with higher levels of sanding. This would be due to the woodgrain already being tighter and the separating ridges being smaller.

GlenH7

Posted 2015-03-20T17:45:05.247

Reputation: 631

This reads like a lot of opinion without a lot of supporting references. – James Jenkins – 2015-03-20T18:59:03.790

Are we required to give references? – dfife – 2015-03-20T19:37:11.037

Your not required to do anything, but the best answers have supporting references (plural). – James Jenkins – 2015-03-20T23:29:25.643