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Apr 24 2010

Surface Prep and Finish, Thoughts and Observations

Published by at 6:19 pm under Finishing,Planing,Sanding

Recently a great question was asked about finish surface preparation for stains, oils, and coating type finishes with hand planes. It was regarding the finish a plane leaves, if sanding is necessary, and which grits are appropriate. It’s a great question. Let’s explore the options.

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In many woods, a planed surface will leave a surface quality similar to that of 600 grit finish smoothness, and without burnishing the wood surface. Sanding in this case may not bring much to the table.

Occasionally there are grains that are just too difficult to plane successfully, and so you must sand.

Planing will help wood pores remain open, sanding will often fill them and burnish the fibers so “in the wood” finishes may have some trouble soaking in.

I often follow this ascending grit schedule: 120 – 150 – 220 – 400, I also use 320 but often omit it on softer woods.

If the wood will have a finish that is “in” the wood, smooth is important, but sometimes it is best to sand at 150 – 220, but no finer. Burnishing will foul the woods ability to allow stain in. Staining will raise grain so sand beyond 220 after staining.

If the wood will be finished with a finish that leaves a coating on the surface, sanding the wood super smooth is not constructive. 150 – 180 is fine, then finish a coat, knock the high spots off lightly with 320 or 400, and re coat. Then sand between coats lightly with fine, fine papers. Think 600-grit or finer range. The last coat, you can likely just let dry, depending on the final outcome.

Using a block is important when sanding finishes. You are flattening the finish, it is easy to sand through so be careful there and go really light near the edges and corners.

Other final outcomes; the last coat can be polished using Rottenstone, Simichrome, even auto polishes.

There are different sandpapers for different moments. Getting the feel for this will develop as you watch the finish you like using and how it reacts to what you do with it. Your observations will help you adapt what works best. Eventually you will decide there are finishes you like and others you won’t so much. You may stick to just a few.

If I am working on a blotchy wood, my sense is and observations are that while burnishing the wood may sometimes help me somehow, I have never felt the help was providing consistent results. What I mean is, while the burnishing may be somewhat successful in slowing how some areas of a board take in finish like oils and stains, and how some areas don’t, is still not averaged out enough to look all that great. I still see the blotches.

The reason is that while I am attempting to burnish the places that will wind up looking dark, I am also burnishing the places where it will wind up looking light to some degree. While I may be able to see where I think the problem areas will be, and focus some burnishing, if I try to focus on sanding some areas of my piece more or less than others, then I am not actually sanding everywhere equally and with consistency. My sense is that a consistent sanding is important, likely more important to the overall look than any attempt to burnish a surface as part of the prep or not.

Another point about burnishing. What burnishing is and how it happens is usually what we don’t want to have happen. Often burnishing occurs when we are using the correct grade of sandpaper for the task at hand, but we have allowed the sandpaper to clog with dust, or the grit to become to dull. This means the sandpaper is trying to do something at a higher grade than it’s rating, because all the sharp points have worn off the grit.

Sandpaper needs to cut, not polish. If we use fresh papers or discs, we should not expect too much burnishing at 180 to 220 grit. This is the maximum we want to sand if we intend to stain, because we actually want to let the stain into the wood. Once the stain is in, and the grain raising is done, we can sand more finely if we like, but we have to use care. Stains are usually going to be used in conjunction with a coating style finish, as stains are often made darker than is our goal by using oils with them, so to smooth the wood further would not be worth the effort. When it comes to finish smoothness with coating finishes, our fingers will feel the surface of a coating, not the wood itself.

If intentional burnishing of the wood fibers is what you intend, the best way to achieve this is to stop sanding at a higher grit. If you should ordinarily stop at 180-220 for the usual surface prep, burnishing would be like stopping at 400 or 600 instead.

If the finish is going to be an oil finish, and I mean a fine oil finish, you can stop at 220 for the first coat of oil, and give it a heavy wash. The lack of burnishing will help let the oil into the wood. But once that coat is in, you may proceed sanding finer, and applying more coats of finish. You may even choose to sand in the finish as you apply it in an attempt to fill the pores of the grain some. As you add oil finish coats, you will sand with higher and higher grits, each successively finer than the last. A lot of walnut gunstocks are finished like this. This often has a really deep look.

Some woods will make these choices for us. Blotchy woods are not so easy to use oils and stains with. Oily woods and dark woods tend not to benefit from oil finishes either. These are woods that will benefit from finishing with Lacquer, Shellac, and Poly.

Some woods will be made to look more descriptive, and have contrast added as well as deepening the natural colors by using oils, and since oil finishes are often usually BLO based, the finish will have an amber colored effect. The rest of the finish is mostly a mineral spirits carrier and a varnish that will be carried into the wood grain, allowed to soak a bit and wiped off, rather than allowed to dry on it. It is important to remember that not all woods are helped by an ambering effect, or a darkening effect. They get all they need by just looking clear/wet.

It may not seem like what effect we want from the wood, or which finish we want to use on the wood has much to do with sanding, but my observations are that it has been helpful to understand these relationships of wood variety, surface prep and finish type in ensemble.

If you are planing, then you get some added benefits that come without some of the trade offs in tow. You will get a surface quality equivalent to what 600-grit sandpaper would accomplish, yet with no burnishing; the grain will remain open and unclogged. It will take oils and stains much like sanding to 180 will. If you are using a finish that coats, you are already more than smooth enough.

If you have to make something using woods that tend to blotch, grain selection in the showy areas can help you, but I have found that while I have gotten to where I can spot where I think a spot will look dark, the spots that wind up looking too light are often a surprise.

What I think is the best way to address blotchy woods, is to either consider them paint grade, or seal them with shellac and use a gel stain over the shellac. This will get you a predictable outcome. Also, consider seeing woods that have blotchy tendencies as woods that are best to be part of a non-show sub assembly. Save the showy, more consistent woods for where the looks are.

I hope that fleshes out some of the mysteries a little more. These are just some of my observations and understandings.

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