PolyUrethane finish keeps bubbling


Old 11-13-03, 11:37 AM
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PolyUrethane finish keeps bubbling

Hi All. I'm putting minwax satin Polyurethane (oil based) on some pine trim that i'm working with. I did stain if first (also oil based) and i sand lightly between each coat of urethane, the problem is that i keep getting small bubbles all over the finish. They sand right out, but then of course, i dont' want to sand after my last coat.

It is a little chilly where i'm working (50-55 degrees appx) so could that be it? I do put on light coats, and i smooth with the brush after i apply it... i don't get it. I've done some finishing before and have never had this problem.

Should i try a semi-gloss? I was debating that anyway, but have always heard that satin will retain it's look longer, simply due to the fact it won't show the dings as much later on...

Anyway, any and all advice is appreciated. thanks.
Old 11-13-03, 12:04 PM
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Thin your poly with some mineral spirits. Start at about 5% ms to poly, wouldn't go more than about 1/3 ms to poly. Yes, it pobably is the temp and thinning should help. I thin the poly even when working at higher room temp. Also, you could try the wipe-on poly which basically is thinned down poly.
Old 11-13-03, 03:44 PM
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You may be your own worst enemy here by overbrushing the poly. Try thinning it, as suggested. The temperature probably works in your favor by allowing any bubbles to work their way out before the finish skins over.

Be careful when working with poisonous compounds, paints and paint products that pose a hazard of fire or explosion, chemicals and paints that may be dangerous to the human body when touched or the fumes inhaled, or the dusts that result from sanding, cutting, chipping, shaping, or scraping. Be sure to read all warning and informative labels that are provided or required to be provided with any products or materials that you may purchase, store, or use. Material Safety Data Sheets are available from the vendors of these products so that you may acquaint yourself with the components of these items to assess any risk.

Hope this helps.
Old 11-16-03, 01:40 PM
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I learned about temperature control during a floor refinishing disaster.
No mention was made about application temp in the instructions and when done in the high 70'sF I wound up with a bubbly mess I had to resand.
When I phoned the company to complain they told me to put the finish in and use it out of the refrigerator.
What a difference that made.
Old 11-18-03, 02:25 PM
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Laying it on with very thin coats helps also. You just have to put more coats on, but they dry faster so it's not too bad. Too much on the brush or whatever your using will also cause bubbles.

Old 11-21-03, 06:30 AM
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Thanks guys... I really appreciate the help.

Interesting about putting it in the fridge - I thought my problem was that it was too cold. Probably can rule that out.

On the other hand, maybe i am putting it on too thick. I've never had an issue with it before, but man it's a definite problem now. What a terrible looking finish i've created. I hate to continue because i have lot to do and i'm on my 6th coat with the baseboard. (sanded it down after coat 4).

I did try adding mineral spirits and it didn't seem to help. Is that what you mean by putting on "thin coats"? thinning the liquid, or do you just mean spread it real thin? I was afraid too much spreading would mean over brushing...

Also, I do have another possibility... I noticed after the first couple coats that i had been cleaning the brush with Lacquer thinner, not mineral spirits. I changed right away, but could it have contaminated the urethane? (Note: i am using a good quality brush that says it's intended for oil based urethanes etc.)

I tell you, i'm confounded at this point. Maybe it's a bad can of urethane.

Anyway, thanks for the help. There's an answer somewhere.
Old 11-21-03, 08:47 AM
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I know, I went thru the same thing.

I was told to put on very thin coats and i did.nt use mineral spirits . I mean thin. It worked that time for me after going crazy with the first 3 coats.

I ended up putting on 2 more coats bringing a total of 3 bad 3 good.

I really was globbing on the first 3 coats .I was all told to not brush it on too fast to use slow strokes.

You have to sand it down after every coat with different grades of paper also.

As for contaminating it i could'nt tell you. Try buying a smaller can and try the thin coat with slow stokes and see if that works. If it does just buy a bigger can and be done with it.

I'll tell you one thing. After all that sanding the next time I'll have someone do it for me next time around. What a mess and the clean up,ugh!

PS: I don't recall if I used oil base at the time it was so long ago. I do remember that i also bought a different brand for the last three coats. I bought the really expensive stuff that i do remember.

Ok, I just found the can I use for the last three coats. It's Mini Wax indoor/outdoor Helmsman Spar urethane clear gloss. It does'nt say it's oil base.

I can't find the original stuff i use, but i think it was oil base. I know oil base is harder to use and takes longer to dry.

Good luck!


Last edited by Bluerain; 11-21-03 at 08:59 AM.
Old 11-23-03, 03:40 AM
Furniture Bldr
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In regard to Poly Bubbling

The main reason why the poly is bubbling is because of the following:

When you stir up the can, it creates bubbles which are left in the finish. When you're done stiring the can, let it sit for about 10 mins so the bubbles can settle down.

One other main reason why you get bubbles is because with poly you're only supposed to brush in one direction. By brushing back and forth and at the same time the finish is drying; it causes bubbles to be left.

if you find that you still get some here and there, take some 600 grit sand paper and run it across the finish after what is supposed to be your final coat. Then take mineral spirits to wipe the little dust it creates.


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