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.
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?
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.
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?
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
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)
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)
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.
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.
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.
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.
mike
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.
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'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.
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.
No one seemed to ask a obvious question...Are these problems really air bubbles or are they fish eyes? Fish eyes are caused by a contamination somewhere. Most times I've seen them the cause was cheap sandpaper that contained Silicon Carbide. I haven't used solvent based poly in 15 years + so I'm no expert on the stuff but Minwax is designed for the DYI type person so should be pretty forgiving.
Silicon is a naturally occurring chemical element, whereas silicone is a synthetic polymer, silicone causes fish eye.
Big difference between tiny air bubbles and fish eyes and no mistaking between them. Most times I see fish eyes when refinishing is with people that used aerosol furniture polish like pledge. I now always assume they did and do a final rinse with lacquer thinner and toss any sandpaper I used before I apply a finish.
Once when I varnished a 40' boat gunwale in the sun, I came back to find quarter sized bubbles over the entire 6" railing that required me to sand all the varnish off and start over.
I never use water based poly anymore - just a personal preference based on years of finishing.
When I made custom furniture for a living, I always used some sort of sealer before varnish or poly
I found that using a seal coat - your choice - fills in grain, and prevents bubble problems pretty much - unless you start with bubbles in your poly.
I used to always brush on poly. I've used every kind of brush and pad, and I found it was always better to thin the poly a bit, to make the poly flow.Do not try to use a dry brush, and be careful of how you scrape the excess poly off the brush. I just tap the brush along the edge of the can, after dipping in the poly to keep from creating bubbles in the bristles. When doing a large surface like a table top, it is hard to brush fast enough to get a smooth coating, as the most new poly's tacks quickly. You also have to resist going back and rebrushing mistakes. Bristles even on a good brush are a constant problem. Just brush as quickly as you can with a loaded brush. I alway try to go in one direction each pass,not brush back and forth. Thin coats are better as they don't leave furrows like thick coats do.
Now I almost always spring for spray cans and do several light coats of poly. The ease of application and the smooth quality more than make up the cost for me. You have to get a good brand like Minwax, so you don't get water spits or bad spray parterns. I ocassionally get a run if I get lazy and spray too heavy, so I recommend never spraying a heavy wet coat.I never get bubbles with a spray can. Just spray lightly, and repeat as often as the directions say. Since I don't have a dedicated spray booth, I just find a covered area and spray outdoors - temp allowing, and take my project in and out. I have done an emergency spray job in the basement, because, even though I had a flourishing furniture business, I still sometimes do things like a shade tree mechanic. I can do things I once did as a pro, without all the bells and whistles. It's just slower and harder.
Good luck
After trying various techniques and looking at some youtube videos on this subject, it appears to me the best way to avoid the small bubbles, or nearly all of them, is to move the loaded brush, whether bristle (synthetic or natural) or foam, very, very, slowly across the application surface.
To answer a question from one of those above, I keep the shop temperature fairly constant between 74-79F all day and night, so I don't think, in my case, temp variability is a factor.
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Related Threads
?
?
?
?
?
Woodworking Talk
1M posts
88.3K members
Since 2006
A forum community dedicated to professional woodworkers and enthusiasts. Come join the discussion about shop safety, wood, carpentry, lumber, finishing, tools, machinery, woodworking related topics, styles, scales, reviews, accessories, classifieds, and more!