Monday, June 17, 2013

Rubbing a Finish

Rubbing a finish is an optional step that "finishes" the finish. If you zoom in to a microscopic level, a finish is rough (Note: "finish" here could be shellac, varnish, polyurethane, etc..., in my case it was polyurethane). Once the finish cures, ideally a few weeks if not a month, and becomes as hard as it's going to get, you can sand it and smooth that roughness out.

In Bob Flexner's book, Understanding Wood Finishing: How to Select and Apply the Right Finish (2nd ed.), he writes on p. 205: "No matter how careful you are, you can'y apply a perfect finish" and "Rubbing a finish does two things. It makes the finish feel smoother, and it gives the finish a softer appearance."

To rub a finish, you take sandpaper of increasingly fine grit, some soapy water, and you sand the finish. No really, you sand it. Remember that the higher the grit number, the finer the sandpaper. Starting with at least 320 but more commonly 400 or 600, you drip the surface with soapy water and sand it.

You work your way up in grit levels for a couple reasons, and where you stop is dependent on two factors: how glossy a finish you want, and how durable a finish you want.

Gloss is dependent on a couple microscopic factors. First off, all varnish or polyurethane is gloss. If you buy satin finish, it has had a flattening compound added to it, which just creates unevenness in the microscopic profile of the finish that makes light not reflect as well. Sanding a finish puts scratches in it. The bigger the scratches, the more the reflective nature of the finish is impeded. So as you work your way up in grit (1000-1600 is pretty flat, 2400-3600 is satin, 4000 is a semi-gloss, 6000 + is gloss) you create smaller and smaller scratches, which cause the light to reflect better and better. A true mirror gloss finish will actually reflect images like a mirror.

That is a picture of my finish rubbing sandpaper from left to right 600, 1000, 1600, 2400, 3000, and 4000 grit.

The durability of a finish decreases (sort of) as you go up in grit. As the scratches in the finish get more fine, any ding, dent, or other scratch the piece receives in its lifetime will stand out very obviously. If the finish is a mirror gloss, it shows wear very easily. If its a more stain finish, then minor wear and tear can sort of blend in more.

After you're done sanding the finish, you use a polishing compound and polish it on up. The picture at the top of the post is of the polishing process.

The tables in my pictures are examples of my first shot at rubbing a finish. Because these are tables, I opted to stop at 4000 grit sandpaper, making it sort of semi-gloss. If you could run your hand along the surface, you could tell something happened to it. Especially if you had a before and after example.

Below are some pictures of the table surfaces. The first two are before I rubbed them. The second two are after I rubbed them. The last two are comparison pictures where they've been put side-by-side on the same picture:




It's hard, if not downright impossible, to convey in picture form the difference. Maybe someone with a DSLR and the knowledge of how to use it could, but I'm rocking an iphone, so that's the best I could do.

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