Turning Archive 2006

Subject:
Sanding Sealer as a finish (very, very long)

RussFairfield
>I received an E-mail relative to finishing with Sanding Sealer yesterday. It was asked relative to finishing pens, but a lot of woodturners are using Sanding Sealer as a finish for other things. It is similar to what I often see on these forums, and I thought my answer might be worth posting here for others to read.

THE MESSAGE
I have used Mylands sanding sealer quite a bit.(nitro cellulose) I bought it about a year ago on the recommendation of Jason at Woodturningz. I was looking for something at the time that was easier to apply than CA.

The second step in the Mylands process is to apply their "high build" friction polish, shellac based. The last step is to apply Carnauba wax. Today I was using the sanding sealer and realized I had a great shine after going through the nine grits of MM.

Here's the question:
If Mylands sanding sealer is a nitrocellulose lacquer product, can that be used as a stand alone finish?

Would not multiple coats be considerec a "final finish"?

What's the down side? What am I missing? I have read your article on finishing and don't recall you mentioning it as a finish.

MY ANSWER
This is going to take awhile, and a bit of a prelude is necessary.

There are 4 qualities to a gloss finish - surface gloss, depth, clarity, and brilliance. I have seen brilliance referred to as "sparkle" in a description of a lacquer finish.

To define each of these a bit.
The surface "gloss" is nothing more than the smoothness of the top surface. You can get that on anything from polished bare wood to black paint.

"Depth" is what we get when we see the light that is reflected from both the top surface AND the bottom surface of the finish film. The more light that is reflected from the bottom surface the greater the depth.

One of the requirements for "depth" is that the bottom surface of the film be just as smooth as the top, and the smoother that bottom surface, the greater will be the appearance of "depth". We could cover a rough wood surface with enough finish that we could get a smooth gloss on the top surface, but there would be no depth because the light is scattered by the rough surface of the wood. This is why the wood that was sanded to 2000-grit (12,000 MM) before finishing will always look better than if we had stopped at 150-grit, regardless of the finish or how thick it is.

Depth is often confused with "chatoyance", but they are different things. Depth has to do with reflective qualities of the finish. Chatoyance is the difference in color and reflectivity between end and flat grain. We would call this "blotch" when it was something we didn't want.

"Clarity" is what it is. There is nothing to interfere with the light getting to the wood surface so it can reflect back. That means no color, and no refracting particles in the film, such as would be found in a semi-gloss or flat lacquer. In lacquer terms, clarity is referred to as "water white". You can see the bottom of the can.

"Brilliance" is that diamond like quality that comes from the little bits of light that are reflected back from intermediate planes or variations in the density WITHIN the film thickness. Think of these intermediate reflectors as the difference between a finish that is put on in several layers, as opposed to only one thick coating. I have seen it referred to as the "sparkle" we often get from a lacquer finish. This is the one quality we can get with multiple coats of lacquer, but that is missing from a dipped application.

Add all of these up and you will see that there is nothing that gives the gloss, depth, clarity, and brilliance of many sprayed coats of lacquer. And now we know why Mark Kauder's and Joel Hunicutt's lacquer finishes look so good.

Now for a definition:
A "sanding sealer" is a drying finish, either lacquer, shellac, or varnish, that has something added to it to make it easier to sand and not ball up in the sandpaper, a built-in lubricant if you will. Shellac is often used because it dries faster and is relatively easy to sand by itself. Lacquer has a longer shelf life, but it is more difficult to sand,, and that problem is solved by adding more of the "Something" to make it easy to sand. Varnish is no longer used because it takes to long to dry. You can make your own by adding Talcum Powder to the finish. The commercial finishes used wood flour at one time; but now they are using Steric Acid. a slippery substance that is used in soaps, release agents, etc.

Note that wood flour, steric acid, and talc are opaque. When the surface is sanded down to the bare wood, the finish and those opaque particles will remain in the crevices and scratches in the wood surface, thereby giving it a smoother and sealed surface; and it easy to sand without lubrication because the steric acid is the lubrication. Those opaque particles will mask the wood surface, and how much masking will depend on how much of it is left behind in the wood. In the extreme, the rougher wood surface will have a washed out appearance.

Now, I will answer your questions

"If Mylands sanding sealer is a nitrocellulose lacquer product, can that be used as a stand alone finish?"
Yes it can, but read on.

"Would not multiple coats be considerec a "final finish"?"
Again the answer is yes, but you will only have a surface gloss.

"What's the down side?"
The down side is that you will have only the surface gloss. Depth and brilliance depend on clarity, and what is put in the lacquer to make it a sanding sealer is opaque. IF the surface of the wood is not sanded sufficiently to make it a reflective surface, there can be no "depth", and you probably couldn't tell the difference. However, if the wood had been sanded and coated with a clear lacquer, you could. I would compare it to the difference between looking at a sunlit scene through a clean window, and through the same window with a screen on the outside.

"What am I missing? I have read your articles on finishing and don't recall you mentioning it as a finish."
You didn't miss anything. I have never talked about "sanding sealer" as a finish because it isn't. Only woodturners have tried to make it one.

I see the goal of a finish as doing everything I can to accent the natural picture of the wood grain, and not mask it with something that is opaque. That is why I prefer to use the clear finish or CA glue as the wood filler. All of the finishes we use will sand quite well when they are dried, and fairly well when they aren't dry if we sand slowly rather than at high lathe speeds. The benefit is that there is nothing in either wood or finish film that will mask the wood grain.

In the end, Sanding Sealer might be quicker, but there is no way that it can be better (unless you choose to not sand the wood before finishing).

Whew, that was long, but I knew of no other way to answer your question. I hope I did.

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