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Polyurethane turning white

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Forum topic by Sandro posted 05-08-2010 02:44 AM 12385 views 0 times favorited 5 replies Add to Favorites Watch
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Sandro

6 posts in 2968 days


05-08-2010 02:44 AM

I am painting an old wooden handrail. I gave a couple of coats of stain and a couple of coats of polyurethane. I waited at least 24 hours between each two coats. Everything went well until I decided to give yet another coat of polyurethane. That polyurethane immediately turned white in several places.

I have read that humidity causes polyurethane to turn white. However I didn’t use any water, and I believe the humidity level in my garage is pretty normal. I used one brush for the stain and a different brush for the polyurethane. I clean the brush with thinner each time and save it inside a zip-locked plastic bag wrapped in a paper towel moistened with thinner, so it doesn’t dry.

Could the thinner remaining in the brush have caused the problem? Or is water the only thing that makes polyurethane turn white? And why did this happen only once?

Thanks!


5 replies so far

View SnowyRiver's profile

SnowyRiver

51452 posts in 2948 days


#1 posted 05-08-2010 02:52 AM

Yes, water and thinner will cause the poly to turn white. It could be that the brush was getting more saturated with thinner as you did each coat and it finally showed up. When I clean the brush with mineral spirits I always snap the brush a few times at the ground (outside of course) after cleaning it to throw off any residue and I leave it out in the open so the thinner evaporates off the brush.

-- Wayne - Plymouth MN

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Sandro

6 posts in 2968 days


#2 posted 05-08-2010 03:34 AM

Thanks, Wayne.

Now I am wondering how I can get the white polyurethane back to its normal state.

View Bureaucrat's profile

Bureaucrat

18337 posts in 3120 days


#3 posted 05-08-2010 03:42 AM

The only way I know of to get rid of the white is sanding, bummer.

-- Gary D. Stoughton, WI

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SnowyRiver

51452 posts in 2948 days


#4 posted 05-08-2010 04:46 AM

Sanding may be the best way although depending on how deep the white is, sanding could be quite a job. The only other thing I could think of would be to use a paint stripper and see if that will remove all of the poly, then start over.

-- Wayne - Plymouth MN

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Sandro

6 posts in 2968 days


#5 posted 05-08-2010 06:33 PM

Fortunately the white was very shallow, and it went away with just a little sanding. Thanks guys!

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