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Thread: Sanding the sheen off polyurethane

  1. #1

    Sanding the sheen off polyurethane

    I have a question on polyurethane and sheen levels. I have a staining project that I am going to finish with polyurethane. (Minwax gel stain on birch). So I tested three different sheens on a stained sample piece (Minwax fast drying polurethane Satin, Semi-gloss, and Gloss).

    I lightly sanded between coats on my sample piece with 400 or 600 grit sandpaper to remove bubbles and increase adhesion. After the last coat, there was still bubbles and imperfections. So I spot sanded with an extra fine sanding sponge. It worked to remove the imperfections, but then the sheen was uneven. Any place that was sanded was substantially less glossy than the unsanded areas. So I decided to keep sanding until it was even and I found that I had sanded off all the sheen of all three samples of polyurethane. In fact - I couldn't tell the difference between the three samples of polyurethane when I was done. At first I thought I sanded off 4 coats of polyurethane with an extra fine sanding sponge, but touching it made me realize it was a very smooth hard surface still in place. In fact, I think I liked the zero sheen look the best. It looks like there is nothing on the wood and is nice and smooth.

    So here are my questions for you experts out there.

    1. Is it okay to sand off all the sheen on polyurethane? Does it still protect just as well from scratching and moisture? It doesn't look like its even there, I can only feel it.
    2. If it is okay, does it matter which polyurethane sheen to use? Satin, Semi-gloss, and gloss?
    3. If it is not okay, can I just wipe on a final coat of "wipe on poly" and get full protection of the built up polyurethane but a bubble-less finish as well?

  2. #2
    I've always preferred a low sheen on my cabinetry and woodworking projects. A "Medium Rub" is my favorite. The higher the sheen, the greater the protection but with polyurethane you may be splitting hairs. As long as you have a couple of coats on the project, you should be good.

  3. #3
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    Here's my 2 cents, so remember what it's worth. Anything but gloss has flatting agents in it to reduce the sheen. In sanding you didn't remove the agents but scratched the surface and now the light is dispersed differently. I don't know how much you sanded off but would guess that if you can feel it you're OK. Maybe Scott Holmes will chime in but be prepared for a not so high opinion of poly.

  4. #4
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    >>>> 1. Is it okay to sand off all the sheen on polyurethane?

    Yes.

    >>>> Does it still protect just as well from scratching and moisture?

    Yes.

    >>>> 2. If it is okay, does it matter which polyurethane sheen to use? Satin, Semi-gloss, and gloss?

    Not really but given the choice, I would start with the least glossy--satin. Also, be sure to build up an adequate film thickness so to avoid sanding through the finish down do to bare wood.

    On somewhat a related issue the following may be of interest.

    To make a clear finish less glossy, typically silica is added as a flatting agent. Silica is normally a perfectly clear, very hard mineral (think of very fine ground glass). In and of itself, it WILL NOT materially diminish the clarity of the finish.

    When first applied, a non-gloss finish appears glossy. But as a finish dries, it slightly shrinks. This shrinkage causes the sharp facets of the silica to jut into and through the surface of the finish causing microscopic peaks and valleys. This unevenness, or roughness, disrupts and disperses the organized reflection of light. This same effect is created by sanding scratches produced when a gloss finish is abraded. The sanding has not diminished the clarity of the finish. It has caused the reflection of the light to be more random so the finish looks less mirror-like.

    The appearance of non-gloss is a characteristic of only the very top rough surface of the film. The surface of a gloss finish is perfectly smooth while a non-gloss finish has some degree of "roughness" to it. The roughness, whether caused by sanding or by a flatting agent, is filled in when a new coat of finish is applied. If that new coat is a gloss, the resulting finish will be gloss and the unevenness produced by the flatting agent or sanding is canceled out by being filled in and buried under the new film of finish. If the coat is a non-gloss (silica containing) film, the unevenness of the prior film is filled in and canceled out also. But the unevenness is replaced by a new rough surface when the new coat dries.

    So the bottom line is that with few exceptions, multiple coats of a non-gloss finish do not progressively reduce the clarity of the finish and it is the final coat of finish that determines the final sheen.
    Last edited by Howard Acheson; 11-01-2011 at 1:52 PM.
    Howie.........

  5. #5
    Thanks for the info. I didn't realize that I was "rubbing out" the finish. That made google searching a lot easier.

    So following up, I see some recommend waiting for polyurethane to completely cure (30 days or so) before rubbing it out. But I was essentially rubbing out the finish after 8 hours. What would be the difference of waiting for a longer cure time before rubbing? I really wanted to wait 48-72 hours to finish, not one month.

    And perhaps relatedly, I found at another site (http://www.antiquerestorers.com/Articles/SAL/rub.htm) that says that when rubbing polyurethane you may rub through the top layer in spots and reach the next layer and get witness marks on your work. It did seem that I did this on my sample piece. The same sight listed above suggested wet sanding in oil to slow down the process. Does this sound right?

  6. #6
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    Urethane is added to varnish to make the finish less susceptible to scratches. Poly is also made softer than non-poly varnish again to help it resist scratching. The urethane also makes the finish less clear and causes it to look somewhat "plasticy". "Rubbing out" is essentially scratching, so poly will tend to not accept an even scratch pattern. All oil based fishes dry/cure in two steps. First the thinners evaporate and then the finish begins to combine with oxygen to cure into a well adhered and protective film. This curing process goes on for a long time but is almost complete within 3-4 weeks at normal temperatures. You can keep smelling the finished surface. When the odor is gone, the finish is cured. At that time you can do the best job with your rubbing out.
    Howie.........

  7. #7
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    Quote Originally Posted by Howard Acheson View Post
    >>>> 1. Is it okay to sand off all the sheen on polyurethane?

    Yes.

    >>>> Does it still protect just as well from scratching and moisture?

    Yes.

    >>>> 2. If it is okay, does it matter which polyurethane sheen to use? Satin, Semi-gloss, and gloss?

    Not really but given the choice, I would start with the least glossy--satin. Also, be sure to build up an adequate film thickness so to avoid sanding through the finish down do to bare wood.

    On somewhat a related issue the following may be of interest.

    To make a clear finish less glossy, typically silica is added as a flatting agent. Silica is normally a perfectly clear, very hard mineral (think of very fine ground glass). In and of itself, it WILL NOT materially diminish the clarity of the finish.

    When first applied, a non-gloss finish appears glossy. But as a finish dries, it slightly shrinks. This shrinkage causes the sharp facets of the silica to jut into and through the surface of the finish causing microscopic peaks and valleys. This unevenness, or roughness, disrupts and disperses the organized reflection of light. This same effect is created by sanding scratches produced when a gloss finish is abraded. The sanding has not diminished the clarity of the finish. It has caused the reflection of the light to be more random so the finish looks less mirror-like.

    The appearance of non-gloss is a characteristic of only the very top rough surface of the film. The surface of a gloss finish is perfectly smooth while a non-gloss finish has some degree of "roughness" to it. The roughness, whether caused by sanding or by a flatting agent, is filled in when a new coat of finish is applied. If that new coat is a gloss, the resulting finish will be gloss and the unevenness produced by the flatting agent or sanding is canceled out by being filled in and buried under the new film of finish. If the coat is a non-gloss (silica containing) film, the unevenness of the prior film is filled in and canceled out also. But the unevenness is replaced by a new rough surface when the new coat dries.

    So the bottom line is that with few exceptions, multiple coats of a non-gloss finish do not progressively reduce the clarity of the finish and it is the final coat of finish that determines the final sheen.
    That's a much more scholarly of saying what I was trying to say (and probably more correct). Thanks, Howie.

  8. #8
    Thanks everyone for the feedback.

    I have a related question. For sanding after the first coat of poly I understand that it is just supposed to be a scuff sand of the surface for adhesion. How much sanding is that? I was working on it this morning and I used a 320 grit on a sanding block and with a low amount of pressure went over the surface 3 or 4 passes in the direction of the grain. Is that sufficient? Its not the smoothest but I was going to wait until after a couple more coats before smoothing out more aggressively.

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    I usually do 220 grit and a couple of passes. Just enough to scratch it some.

  10. #10
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    Mark...

    In my world, what you described is fine for scuff sanding. I don't use a block, I've gotten the "feel" dialed in - "light pressure" is "almost no pressure" to me, but that is being picky - the point is to get it smoothed out. I also use 320g, but others differ, like Jim above.

    One other piece of reference info.......I brush on varnish. When I get "done", I will do one final sanding to get it nice and smooth, and then do one last wipe-on coat. I'm not a poly guy, so I can't tell you if there are any eccentricities there, but with varnish the wipe on coat is 50/50 varnish and thinner. One last, light, thinned coat goes on very smooth - no tromping around the shop - and it very quickly [30 min or less] has set up to where the dust is not a factor, and the final surface is great.
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  11. #11
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    In between coats sand with 180 and 220 if solvent based material and up to 320 if waterborne.
    I have always sprayed gloss poly and then rubbed it out to the sheen the customer ordered based on my samples. In a production situation you would spray the sheen you want, no rubbing.

  12. #12
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    Using a gloss varnish (poly or not) will help keep the richness of the wood highlighted. Rubbing out the last coat to the desired sheen with wool or a non-woven pad, then waxing will finish up the project. Scuffing between coats will help adhesion.
    Fully cured varnishes will buff out well. Don't rush the job.
    As was stated earlier, production work will require a different approach as well as different finishing mediums.
    Bill
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