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Finishing the finish
Rubbing out a finish

Sheen - how is light reflected from the surface

There are lots of names for the amount of light reflected from the surface, but the most common (unencumbered by marketing hype) seem to be

The final sheen is determined by how much the reflected light is diffused, achieved in either of two ways...

Finishing and rubbing sequence

I like to rub out my finishes, since it allows me to hide the imperfections in my spraying technique. But I learned the hard way that it is much harder (or some would say, you can't) to rub out a finish that has the "grunge" to diffuse the light. Therefore most of my work currently is applied as a gloss finish (no grunge), and then rubbed with various abrasives, to the desired sheen.

Filler

If I want a very smooth surface, whether satin or gloss, the grain must be filled. With an open grain wood, like oak, I may want a very smooth surface on part, like a desktop, while leaving the rest of the piece with the characteristic open grain. For the smooth surface, I use a separate wood filler. It can be oil or water based; many use pummice or other fine-ground mineral as the actual filler, so a quart of filler may be much heavier than a quart of paint. My favorite is Behlens water base grain filler, in either natural (light) or dark tone. It is spread across the grain and rubbed in, dries quickly, and is easily sanded (sand off all the filler on the surface). It takes stain well, but as a mineral, does not color well with dyes. (See Stain vs. Dye.)


Sanding sealer

If the wood is smooth, with minimal open grain like cherry, the first coat of finish can be a "sanding sealer." This can consist of a special sealer, a coat of shellac, or in some cases can be the first coat of the final finish. A second function of the sanding sealer is to bring out the grain and color of the wood. A special sanding sealer is typically soft and contains extra chemicals (often stearates, nicknamed "soap"), to allow easy sanding without clogging the paper. After the pores are filled, the bulk of the sealer is sanded off - even down to the surface of the wood, leaving a perfectly level surface. The stearates in many special solvent-based sanding sealer products interfere with water-based finishes, so other sealers (or a barrier coat of dewaxed shellac) must be used after the sealer and before the water-based finish. My favorite sealer is Target Coatings EM1000 water-based sealer - it applies and sands well, and "pops" the color of the wood. My primary finish (Target EM6000 acrylic lacquer) leaves a powder when sanding (does not clog the paper), so can be used as a sanding sealer, especially if thinned slightly to get into the wood pores. (However using this water-based lacquer as the sealer doesn't pop the color of walnut as well as other water or solvent based sanding sealers.)

Special note on Shellac: I have routinely used Zinsser pre-mixed wax-free Seal Coat as both a sanding sealer and as a barrier coat between mystery finish and my final coats of water-based finish, and have never had a problem. Some of my friends have had trouble with this procedure. One recommendation is to use freshly mixed shellac (crystals and alcohol) but I have never tried that approach, since I have never had a problem.

Build coats

After a perfectly level and smooth surface is created, then the "build coats" need to be applied. If you use a thinned finish (diluted lacquer or a wipe-on varnish) then a large number of thin coats are used. Your spray or brush technique determines how many coats are required. Generally I do little beyond the first coat on the outside back of a cabinet. The inside back can be seen but has virtually no wear, so it may only get one or two build coats after the sealer. A counter or shelf may get 2 quite heavy coats (if conditions are perfect and I am "on my game") but more likely will get 3 to 5 thinner coats. When I am almost done, I go over the surface quickly with fine sandpaper (400 grit or finer), and apply the final coat.

Final Coat

If the grain is closed or filled, so the surface is perfectly smooth, then I normally apply a gloss final coat, and reduce the sheen (if required) by sanding. However, if the grain is open and a satin or semi-gloss finish is required, sanding doesn't work by itself... the finish in the bottom of the grain pores is shiny. In that case, I either apply a final coat of satin finish, or rub the finish with an abrasive that will reach into the pores (like the white 3M abrasive pads), either hand operated, or stuck to a ROS.

Rubbing out a finish

If you are going to rub out the finish, you should use gloss material. Some people argue that all the build coats of finish should be gloss, even if you are going to use a satin or other sheen for the final finish, so the "grunge" doesn't obscure the grain (a subtle point, but I have diagnosed several cases of "cloudy" finish as too many coats of satin). In either case, the bulk of my finishing is done with gloss, which I buy in bulk, and which requires less stirring and filtering.

P400 grit abrasive does a good job at leveling a finish, without excessive removal of material (we are trying to build the finish, remember!) Then, a quick "once over" with 600 grit leaves a nice satin finish. I may go to 800 grit if I want a slightly higher semi-gloss finish.

If the target is a high gloss finish, start by "sanding" the finish with 1200 or finer. If that sanding "knocks off" any imperfections, you are done. But if the finish is still rough, they you need to level it with 400 grit, and work you way back up to 1200 grit or finer.

I have not found inexpensive pads for machine sanding that are finer that 800-1000 grit (the quick sanding mentioned above is done with expensive sponge-back sanding pads, sometimes kept cool and clean with a mist of water). Therefore I sometimes switch to rubbing compounds. I have not tried a lot of options, but had several recommendations for the Menzerna compounds, applied with Surbuf pads on my ROS. This works well for me.

I started with Menzerna compounds because they were recommended as working with a variety of finishes, without side effects. It seemed everybody sold them as well. Lately they seem to be less widely available and several people have asked me where you can get them. First, they are German, and in addition to wood, are also used in metal, plastics, and auto finishing, so may be available locally at auto finishing suppliers. I bought mine at Jeff Jewitt's Homestead Finishing Products.

Most of the film finishes dry in hours or days, but take weeks to fully cure. I tell my customers that it is like concrete... after a day, you can walk on it, and kids can't put their initials in it, but it is far from full strength... use it gently, but wait a few weeks before sliding a hard object across the surface, or putting a heavy metal lamp on the new tabletop. Some experts argue that it should be fully cured before rubbing out the finish. I have not had a problem with dry sanding soon after the finish is dry to the touch, but the finish does need to fully cure before you go to wet sanding or rubbing compounds.

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