Polyurethane bubbles driving me crazy - Woodworking Talk - Woodworkers Forum
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post #1 of 13 Old 12-30-2017, 01:20 PM Thread Starter
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Polyurethane bubbles driving me crazy

I am going crazy trying to "finish" a project. I am getting bubbles in my oil based polyurethane, and I don't know why. I've watched hours of videos on U-tube, read countless articles, tried many techniques, but am still getting bubbles. No, I'm not shaking the can, and I did buy a good brush, and I am applying slowly and not brushing vigorously. My first few coats were full strength poly, and I think I was sanding too much in between coats, so I tried sanding lighter, then i diluted poly with some paint thinner. This went on much easier, and even though I saw thousands of bubbles at application, I was hoping most would pop during drying because of the thinned solution. The next morning, the bubbles were bad, and kind of streaky (see picture) If anybody has any suggestions, it would be greatly enormously appreciated. Thank you.
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post #2 of 13 Old 12-30-2017, 01:25 PM
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Thatís very strange. Never seen anything like that. What brand of poly are you using? Iíve been using Minwax oil based poly quite a bit lately and have had no such issues. I use a Purdy brush and brush it on thick and vigorously. I do get minor bubbles in the application, but they disappear quickly. I always steel wool in between coats and that takes care of any bubbles that didnít pop.


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post #3 of 13 Old 12-30-2017, 01:33 PM
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Are you using a stearated sandpaper (white) to sand between coats? This could cause a problem.

Generally when you get bubbles in a finish it is from applying the finish too thick. When you brush a finish you use as soft of a paint brush as you can and apply the finish as thin as you can with as few strokes as you can. The finish begins to set up as soon as the brush leaves the can and brushing too much gets more air in the finish causing it to set up all the faster. If it's thick the finish can skim over the surface with air trapped underneath and cause bubbles. For the most part it just makes more brush marks in the finish. The finish needs to lay on the surface and flow together letting the brush marks go away.

If you have compressed air a twenty dollar sprayer from harbor freight can make most of your finishing problems go away. Myself I spray everything. The only time I finish with a brush is when I'm in a customers house and overspray is an issue. Spraying is just so much easier and will look better than you could ever do with a brush.
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post #4 of 13 Old 01-02-2018, 01:59 PM Thread Starter
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This is what I hear from everybody..."I've never had any issues with bubbles, and I apply fast, don't thin, etc., etc."
If I thought this was something only pros could do, I would give up, start over, and get a sprayer, but I know there is something (probably minor) that I am overlooking. I am using Minwax fast drying poly. Could dust be causing this many bubbles? Could poly residue on sandpaper be causing issues?
Any help would be greatly appreciated.

Thanks.
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post #5 of 13 Old 01-02-2018, 02:10 PM
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if it is the Minwax fast dry, try a foam brush. apply it quickly, and take ONE leveling stroke*. I think you may be working it too much.


*move the brush from dry to wet.
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post #6 of 13 Old 01-02-2018, 04:39 PM Thread Starter
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Based on much research, it seems that most say a foam brush will give many more bubbles. I have been putting poly on with one brush stroke, and tipping off with one stoke. I can't imagine that is too much. I can try a foam brush, but why would that have less bubbles? I'm sure I would still need to use 1 stroke and 1 tipping stroke.

Thanks for all help and suggestions.
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post #7 of 13 Old 01-03-2018, 09:07 AM
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couple of more ideas.
1. with winter here, make sure the wood and the poly are warmed up, at least above 60 deg. warmer is better.
2. don't blow any air around as it is drying. it may be drying too fast before the bubbles work their way out
3. don't shake the can before using - just stir gently. maybe let is sit for 15 min before applying.
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post #8 of 13 Old 01-04-2018, 12:24 PM
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Red Oak has an open grain. Try using a grain filler after staining, then apply the finish the way you have been and see if you get bubbles.
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post #9 of 13 Old 01-06-2018, 10:06 PM
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I have found cold wood is my biggest air bubble producer. As it warms up the air in the wood will expand and must escape. As it leaves the wood, it will push its way through the poly finish leaving bubbles. To avoid this, make sure the wood is totally warmed up before applying any poly finish.
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post #10 of 13 Old 01-06-2018, 11:36 PM
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Quote:
Originally Posted by tewitt1949 View Post
I have found cold wood is my biggest air bubble producer. As it warms up the air in the wood will expand and must escape. As it leaves the wood, it will push its way through the poly finish leaving bubbles. To avoid this, make sure the wood is totally warmed up before applying any poly finish.
It might be your finish has thickened because of the cold. You can heat the finish up to about 80 degrees and apply it to cold wood. This would allow you to spread it out thinner.
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post #11 of 13 Old 01-08-2018, 11:14 AM
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Quote:
Originally Posted by tewitt1949 View Post
I have found cold wood is my biggest air bubble producer. As it warms up the air in the wood will expand and must escape. As it leaves the wood, it will push its way through the poly finish leaving bubbles. To avoid this, make sure the wood is totally warmed up before applying any poly finish.
I don't know for sure what is causing the bubbles...but I immediately thought of just what the gentleman above said. What is the temp in your shop? Is it maintained there OR are you firing up a heater only when you are working in it and other times it gets cold, etc?
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post #12 of 13 Old 01-08-2018, 11:10 PM
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IowaDave....As I read your post, I realized I wasn't very clear in my above post. Here in Mich in the winter months is when I work in the wood shop. yes, it is freezing and I don't keep heat in the shop 24 hrs a day. So when I turn on the heat in the shop, the shop heats up much quicker than the things in the shop including the wood projects. Seems I'm always in a hurry to get working and I will put poly on the project, and before it dries, the project heats up, expanding the air in the wood forcing air out of the fibers, causing a bubble in the poly. Just my experience.
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post #13 of 13 Old 01-16-2018, 11:05 PM Thread Starter
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So, I think I figured out my problem, and I want to try to explain briefly in case any one else is having similar issues. Initially I think I was sanding too hard between coats. I think I was sanding most of the varnish off. Therefore I wasn't seeing less bubbles with each coat like you should expect to see.

In search of the answer, I did many things. One such thing was thinning poly with paint thinner. Although this probably gave the bubbles more time to settle, I think it allowed more dust to collect on the poly. This made the surface seem very rough to the touch. I was frustrated thinking I did not solve the problem until I realized it was most likely the dust that was causing the rough texture.

So in addition to sanding much more lightly, I took extra precautions to have a dust free environment. Finally, I still had a little bit of roughness to the touch after the final coat of poly. The advice I received suggesting sanding with a paper bag worked great. This took out the roughness, and after applying a little bit of paint thinner to shine, I'm happy with the results.

There is still room for improvement, but my advise for a non professional that is having similar issues based on all of my research and experience is:

get a decent brush that is made for oil based poly (if you are using oil based poly), apply poly thinly and use slow brush strokes. Thinning poly with paint thinner or mineral spirits gives bubbles more time to pop, but be careful as this allows more dust to settle on work. Sand super lightly between coats (just enough to make work feel smooth to touch) Make sure wood is warm. Warmer is better.

Thanks everybody for all of their help and advise and I hope I can save somebody else from pulling out all of their hair.
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