how to keep those small bubbles out of Minwax semigloss polyurethane finishes - Woodworking Talk - Woodworkers Forum
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post #1 of 24 Old 08-13-2020, 03:33 PM Thread Starter
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how to keep those small bubbles out of Minwax semigloss polyurethane finishes

I've been using Minwax clear semigloss to finish some new (unfinished new wood) white oak tables and I'm having difficulty keeping small bubbles out of the finish once the polyurethane dries. I am hand-brush-finishing using good brushes (both natural bristle and synthetic bristle), I brush slowly to avoid creating the bubbles, and I always filter the polyurethane through a paint filter prior to application. I cannot spray since I do not have a hood or the equivalent. Most of the small bubbles burst before the polyurethane fully dries, but not all of them. One of the tables I am making of white oak I have stained black (TransTint in dewaxed shellac) and I am concerned that the bubbles from the polyurethane applied thereafter might show more on a black background. I am using polyurethane because the finishes on these tables need to be durable. Any thoughts about how to keep these small bubbles away.
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post #2 of 24 Old 08-13-2020, 05:34 PM
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How many brush strokes do you use to apply a layer? I was taught that the more brush strokes you use to apply polyurethane finishes, the more tiny bubbles you create. Ideally, you should apply polyurethane using the best quality synthetic brush with one brush stroke, two at most, then stop. Don't worry about the smoothing out the ripples - gravity and flow will settle them before the finish cures. Does that help?
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post #3 of 24 Old 08-13-2020, 05:40 PM
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Try using a good foam brush. Are you gently stirring the poly...aggressive stirring can introduce bubbles.
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post #4 of 24 Old 08-13-2020, 06:08 PM Thread Starter
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I have only applied a couple of brush strokes to each surface area, and I gently stir the polyurethane in the can to avoid creation of bubbles.
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post #5 of 24 Old 08-13-2020, 06:45 PM
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I prefer wipe on poly which is just a thinned down version. It takes more coats but I find it easier to get a good result. Another option is to do a couple of regular coats with a brush and after sanding do a couple of wipe on coats.
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post #6 of 24 Old 08-13-2020, 07:43 PM
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It sounds like it’s not your brushing technique.

I think it that when you apply finish on bare wood, as the finish enters the pores, it displaces air which rises to the surface. If the finish has begun to dry, it becomes too thick for the bubble to pop. In theory, this should only happen on the first coat or two and you should probably be sanding between coats anyway.

Alternatively, you can thin the finish a little bit to slow the dry time so the bubbles can pop. I think it’s pretty normal to thin the first cost or two.

BTW, are you using water based or oil based? I’ve always found Minwax Poly to be very forgiving. Is it unusually warm where you’re finishing?
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post #7 of 24 Old 08-13-2020, 08:01 PM
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I used foam for years but a good brush works better. I generally don't like applying brush on poly when it is too hot (dries too fast), if I have too I do what terryh mentioned with a brush/sand and then wipe on
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post #8 of 24 Old 08-14-2020, 01:41 AM
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With oil based polyurethane I thin 25% to 50% and wipe on.
Don't shake the can to mix but rather stir gently.
I store the thinned polyurethane (MinWax) in canning jars. Generically called Mason Jars but more recently manufactured by Ball. What I have noticed, as the jars are clear, the mineral spirits don't seem to separate and doesn't need to be remixed.

I can buy the canning jars at my local Ace hardware. (Springdale Ace Hardware)

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Just a dumb old paper boy from Brooklyn, NY
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post #9 of 24 Old 08-14-2020, 06:54 AM
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If it is oil based poly it should be natural bristle brush. I use a lot of Minwax poly on the interior trim I am adding since that is what the builder used on the trim when the house was built and I am not having any bubble issues. I think it is your application technique.
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post #10 of 24 Old 08-14-2020, 03:43 PM Thread Starter
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I think part of the problem is I'm using a very good (and expensive) 3" natural bristle brush that I've been using to apply Minwax semigloss polyurethane (not water-based), multiple coats, on several large tables over several months and have been keeping the brush (the bristles part) immersed in terpentine (in a covered glass container) with the handle suspended via a wire through a hole (I drilled) in the handle so the bristles come nowhere near the bottom. This has worked well for nearly several months and saves me much time not having to clean the brush after each application and after each table. I simply leave the brush immersed between uses. However, it may be that the brush is now reaching the end of its functional utility. Therefore, this morning I tried a 2.5" brand new high quality synthetic bristle brush, and though it also left some small bubbles, it did not leave as many, and for those it did leave I gently swept the brush over the bubbles/wet polyurethane (on the table) at an acute angle applying no pressure at all (relying only on the brush weight) across the entire length of the grain on the table, and this eliminated nearly all of the small bubbles and delivered an even coat like always. I will try this approach again with the next coat.
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post #11 of 24 Old 08-15-2020, 06:10 AM
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Quote:
Originally Posted by NoThankyou View Post
With oil based polyurethane I thin 25% to 50% and wipe on.
Don't shake the can to mix but rather stir gently.
I store the thinned polyurethane (MinWax) in canning jars. Generically called Mason Jars but more recently manufactured by Ball. What I have noticed, as the jars are clear, the mineral spirits don't seem to separate and doesn't need to be remixed.

I can buy the canning jars at my local Ace hardware. (Springdale Ace Hardware)

I do the same, but I also lay a piece of plastic wrap on top so it keeps the cap from sticking to the threads.
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Last edited by redeared; 08-15-2020 at 06:12 AM.
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post #12 of 24 Old 08-15-2020, 02:32 PM Thread Starter
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Using the approach I described above seems to have worked with the second coat; very few small bubbles. I again used a new good quality 3" synthetic bristle brush (again, this was a brand new brush never used before, i.e., a second new brush) and after applying the Minwax semigloss polyurethane and gently spreading it out the length of the wood grain about 2 brush widths for each swath, I very lightly and very slowly dragged the brush at an acute angle relying only on the weight of the brush. The result (3rd coat) has dried and I think turned out well. There are almost no small bubbles. However, using new brushes for final coats I think is a little costly. This particular table is stained black with the polyurethane applied (3 coats) on top, so if there are many small bubbles, with the black background they show up fairly visibly when the finish is dry. This also happens when applying to new unfinished white oak tables I have recently done that are not stained (not black), but the bubble are more apparent with the black background. The method I describe here has provided a finish on the black background that is acceptable to me; I think it looks good.
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post #13 of 24 Old 08-15-2020, 05:33 PM
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Quote:
Originally Posted by cassidy View Post
Using the approach I described above seems to have worked with the second coat; very few small bubbles. I again used a new good quality 3" synthetic bristle brush (again, this was a brand new brush never used before, i.e., a second new brush) and after applying the Minwax semigloss polyurethane and gently spreading it out the length of the wood grain about 2 brush widths for each swath, I very lightly and very slowly dragged the brush at an acute angle relying only on the weight of the brush. The result (3rd coat) has dried and I think turned out well. There are almost no small bubbles. However, using new brushes for final coats I think is a little costly. This particular table is stained black with the polyurethane applied (3 coats) on top, so if there are many small bubbles, with the black background they show up fairly visibly when the finish is dry. This also happens when applying to new unfinished white oak tables I have recently done that are not stained (not black), but the bubble are more apparent with the black background. The method I describe here has provided a finish on the black background that is acceptable to me; I think it looks good.
That's why foam brushes were created.
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post #14 of 24 Old 08-15-2020, 06:24 PM
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I have a question. Did you apply the finish in the morning and did the temperature rise as you worked?
I realize you are working with a varnish but the problem with bubbles is prevalent applying epoxy.
Try warming the surface with a heat gun or hair dryer first. Then apply the finish. The bubbles may not appear.
Epoxy, which you are not using , will develop bubbles on rising room temperatures. Possibly the varnish may have the same type of attributes that make it prone to bubble. The brush you are using in my opinion has nothing to do with bubbles. Worth a try.
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post #15 of 24 Old 08-16-2020, 01:05 AM
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I totally agree with the posters that advised wiping on either "Minwax Wipe-on Poly Finish" or much cheaper, thinned Minwax Poly. No bubbles, no expensive brushes to clean, no sponge brushes to toss and a beautiful finish. Since it's thinned you will need to apply more coats but it is so easy and error free it doesn't matter.

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post #16 of 24 Old 08-16-2020, 01:35 PM
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I'm not a painter or refinisher by any stretch of the imagination. I do experiment and test all the time. I don't know if this would work and there are probably many here who would know better than me regarding what I'm about to say.

I see guys apply epoxy and then to remove the bubbles they lightly go over the epoxy with a heat gun or torch. Of course, I would experiment on waste if I were to try this but maybe it would work for you? I'd be interested in hearing if anyone on this forum has tried the technique on poly.
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post #17 of 24 Old 08-17-2020, 06:08 AM
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Originally Posted by denisd View Post
I'm not a painter or refinisher by any stretch of the imagination. I do experiment and test all the time. I don't know if this would work and there are probably many here who would know better than me regarding what I'm about to say.

I see guys apply epoxy and then to remove the bubbles they lightly go over the epoxy with a heat gun or torch. Of course, I would experiment on waste if I were to try this but maybe it would work for you? I'd be interested in hearing if anyone on this forum has tried the technique on poly.

I would never consider doing that, epoxy is a completely different animal
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post #18 of 24 Old 08-17-2020, 06:18 AM
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Originally Posted by JIMMIEM View Post
That's why foam brushes were created.

I use the same brush over and over many times, just clean it well with mineral spirits, and then soap and warm water. I got tired of buying foam brushes, their fine if your doing one off stuff but for me their not practical and costly
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post #19 of 24 Old 08-17-2020, 10:26 AM
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Our local finishing expert told us to avoid foam brushes. He says that they tend to create more bubbles. He prefers high quality bristle brushes - with proper care, they can last a long time. He says that one of his favorites is 20 years old.
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post #20 of 24 Old 08-17-2020, 10:58 AM
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Quote:
Originally Posted by Tool Agnostic View Post
Our local finishing expert told us to avoid foam brushes. He says that they tend to create more bubbles. He prefers high quality bristle brushes - with proper care, they can last a long time. He says that one of his favorites is 20 years old.
No right or wrong answer.....just different opinions and preferences. I was using an oil based flooring polyurethane and the manufacturer's customer support rep recommended the foam brush.
What ever works for you.....circular saw blade left vs blade right.
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