View Full Version : bubbles in polyurethane
06-07-2001, 06:30 PM
Is there any way to completely eliminate the bubbles when applying polyurethane. I've already applied five coats to my bar top and have done everything by the book. Sanding between coats, tack cloth, patience. Still getting very small bubbles.
06-07-2001, 07:05 PM
One of the reasons I don't like the stuff. But, here iare two tips that might help. Use a good brush and only dip it into the pan about 1/4 of the way up the brush.
Apply, with drawing the brush only in one direction overlapping a small amount.
The second tip is thin it a bit before you apply. This will help to allow the poly to flow better.
06-07-2001, 08:21 PM
Two words: foam brush
I second Lou's suggestion of thinning it. I normally put all but the last coat cut 50%-50% with mineral spirits.
Also, how are you stirring? By hand I hope - not power/not shaking.
06-08-2001, 12:34 AM
Is the POLY your using water based or oil ????/ read the instructions carefully ..as in some poly's u ARE NOT suppose to mix or shake at ALL>>> good luck
06-08-2001, 06:45 AM
Use a good quality 3" or 4" wide chisel head brush with feathered tips. (If I was a chick I'd say feathered tips look like hair with split ends. But since I'm not....)
06-08-2001, 11:46 AM
Mark says foam, I have never had good luck with them but, He has. If you want to use a real brush? If it is a oil based finish use a good quality natural brisel brush. The best for furniture would be about a 2 inch bager brush. Very expensive. This would only be used for oil based varnish or poly. Clean it well and store it in the orginal sleeve tht it come with.
If it is a new water based poly use a good quality nylon brush and use the same clean and store instructions above. Keep this brush for just use on clear finish. Don't let someone, like my kids do use it to paint some sign or room with latex. you will be buying a new brush when you need to put down your next coat of poly.
06-08-2001, 02:07 PM
When I say foam brush, I'm not talking about the 12 for $1 bulk packs with the wooden handles (or the cheapie red handled ones) - those essentially dissolve in poly. But if you buy the better grade (much finer and denser foam) brushes for about $1 each (white handles shaped like a 'real' paintbrush), they'll give a nice finish.
06-08-2001, 11:50 PM
I thin poly with MINERAL SPIRITS about 50-50. The first coat usually requires some sanding. Usually it is best to wait a day and a half between coats. Wipe the surface before each coat with a cloth with mineral spirits. (Picks up most of the settled dust and left over saw dust.)
As Mark says, foam burshes and gently, ***VERY*** GENTLY apply in one direction only. Don't use an overhead light to see what your doing. Use a light 10 feet away and look at the surface at an angle so that you can see the defects and missed spots. If you get a bubble, gently brush the surface again and sweep the bubble off.
Now questions for the thread.
Regarding water based poly?
Do you thin water based?
Is distilled water necessary for thinning?
Is the final result with water based as durable as oil based?
Does anybody have anything good to say about water based poly?
06-09-2001, 01:50 AM
I don't have anything good to say about any poly. Using it was something that almost caused me to give up woodworking. All other finishes I have found to give better results with less work and less problems.
And the others don't end up looking like a plastic finish.
Mike Mc (Guest)
06-10-2001, 12:11 PM
i beleive that your urethane is contaminated with air. When brushing a finish pour a small amount of finish into a different clean conainer. Use a good brush, dip the brush abour 1/4 of the way into the container and tap the brush on the side of the container. Do not wipe the brush on the side of the container, this introduces air into the finish. Air in the finish leaves small bubbles. (The small bubbles that you are experiencing.) After applying the finish do not return the used finish to the origional container. This will only contaminate the finish.
Tom Hintz (Guest)
06-20-2001, 08:22 AM
Two thikngs I have found that help are,
Making sure I do not have a fan on in the shop. That seems to force the poly to "skin over" before it lays down, retaining any surface deviations.
Second is a technique (I used to be a sign painter) called "laying off" the coat of paint, poly, whatever. You can do it with any brush or roller.
Apply the coat of whatever you are using, then go back over it with an reasonably dry brush (or roller) in the direction of the grain, but dragging the brush as lightly as possible across the surface. This breaks bubbles and flattens other undried high spots.
It takes a little practice, but once you get the feel for it you can produce glass-smooth finishes with many kinds of materials.
06-24-2001, 12:09 AM
I am a professional woodworker.for the past 5 years i use water based poly by Carver Tripp..the only place around my town that sells it is Menards.it is called Super poly ..designed for floors .i use it on all the bars i make..i put 5 coats on..being that it is water based the first coat raises the grain slightly so i lightly sand after the first coat with 220 grit..water based does not yellow and it far surpasses laquor or oil based polys in durability...i spray it on with a HVLP 2 quart remote spray gun from APOLLO..it dries fast in about 15mins.between coats.
07-19-2001, 07:57 PM
Sounds funny...but...I've been using poly for years, with foam brushes too...I thin the poly slightly, brush it on...then blow on the area you just coated...bubbles disappear! I'm told that it's the carbon dioxide in the breath that bursts the bubbles - whether that's true or not - it works for me - I don't get any bubbles at all.
Russ Ramirez (Guest)
07-20-2001, 10:01 AM
The easiest way to eliminate bubbles is to soak the brush you are using in a compatible thinner - mineral spirits for oil-based coatings and water for waterborne coatings. The reason for the bubbles is that the wicking action of the bristles draw air from the dry bristles into the bristles saturated with finish. Soaking a brush by submerging it past the metal ferrule and waiting until the air bubbles stop coming out of the ferrule area is the right amount of time to soak it. Remove the brush an squeeze the excess solvent out of the brush, then proceed as usual with applying the coating. Makes complete sense when you think about it and also makes clean-up much easier.
07-20-2001, 10:39 AM
I have used foam applicators before and really liked how they worked for me.
My favorite is satin finish poly.
When thinning water poly, I would suggest RO water or distilled. Never tap water, at least not without some sort of filtration. We have an RO system on our sink and I use it for any water based thinning I do.
I think it's time I get a HVLP sprayer. I do like my sipon/pressure feed gun. But I think the HVLP might get more on the work and less in the wind.
Soak the brush to the hilt first....I'm gonna try that. That might even help with the spar varnish I'm currently using. But I haven't had any bubble troubles.
Blowing on the finish to pop the bubbles...sounds reasonable to me. But my breath might curdle it. LOL
Lots of good thought here....
"Precision Firewood Specialist"
07-20-2001, 10:42 AM
...you desirve a good shelacing!
"Precision Firewood Specialist"
Douglas Scott (Guest)
07-20-2001, 10:53 AM
You might want to get a good book on finishing. I have Bob Flexner's Understanding Wood Finishing, I am learning a lot from it. I like his writing style and he makes things easy to understand.
You might try thinning the poly, or using a retarder to slow the cure giving more time for the bubbles to disperse. I have thinned it 50-50 with naptha and wiped on with a rag, dries faster and no bubbles, requires more coats.
07-25-2001, 11:45 AM
>dilute poly 20% to 25%
>brush SLOWLY, re-load brush as soon as it stops depositing varnish
>don't overwork the finish, or you will get a sea of bubbles
>when surface is covered, very lightly drag the tip of your brush over the surface lightly in one direction
>nothing wrong with foam brushes in my book