# Finishing and Humidity



## Nick W. (Jan 30, 2018)

What are the consequences of applying stain and polyurethane in high humidity? Is it just a prolonged drying time?


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## Steve Neul (Sep 2, 2011)

It depends on how high the humidity is. If you were using a fast dry finish you shouldn't apply the finish if the humidity is above 70%. An oil based polyurethane is a slow drying finish which is a little more tolerant. You would be safe to use it up to about 78%. High humidity will greatly extend the drying time. Where in warm dry weather the finish might dry enough for another coat but when the humidity is up you probably should wait 24 hours. 

What happens is moisture from the air can get into the finish and make it cloudy. With slower drying finishes there is time for the water to come to the surface and get out. If the humidity is really high not all the water will escape and will cloud the finish. 

The humidity doesn't make any difference when using stain.


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## Nick W. (Jan 30, 2018)

Steve Neul said:


> It depends on how high the humidity is. If you were using a fast dry finish you shouldn't apply the finish if the humidity is above 70%. An oil based polyurethane is a slow drying finish which is a little more tolerant. You would be safe to use it up to about 78%. High humidity will greatly extend the drying time. Where in warm dry weather the finish might dry enough for another coat but when the humidity is up you probably should wait 24 hours.
> 
> What happens is moisture from the air can get into the finish and make it cloudy. With slower drying finishes there is time for the water to come to the surface and get out. If the humidity is really high not all the water will escape and will cloud the finish.
> 
> The humidity doesn't make any difference when using stain.




Will the humidity affect lacquer like polyurethane?


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## Steve Neul (Sep 2, 2011)

Nick W. said:


> Will the humidity affect lacquer like polyurethane?
> 
> 
> Sent from my iPhone using Tapatalk


Lacquer is much more sensitive to humidity than polyurethane. The good thing about lacquer is if you spray it and the finish blushes you can stop and wait for drier weather and when you spray the next coat it re-wets the dried finish letting the water out. With polyurethane if it blushes it's just there permanently. You have to strip the finish off and start over. Now with lacquer if the humidity is borderline of being too humid to spray you can add a retarder thinner to the lacquer which slows the drying time down so it won't blush. Retarder thinner really only gets you to about 72%. Personally when the humidity reaches 70% I just stop finishing if at all possible. Once you start adding retarder thinner it not only makes it take longer to dry it makes it take longer to fully cure and harden properly.


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## Nick W. (Jan 30, 2018)

Steve Neul said:


> Lacquer is much more sensitive to humidity than polyurethane. The good thing about lacquer is if you spray it and the finish blushes you can stop and wait for drier weather and when you spray the next coat it re-wets the dried finish letting the water out. With polyurethane if it blushes it's just there permanently. You have to strip the finish off and start over. Now with lacquer if the humidity is borderline of being too humid to spray you can add a retarder thinner to the lacquer which slows the drying time down so it won't blush. Retarder thinner really only gets you to about 72%. Personally when the humidity reaches 70% I just stop finishing if at all possible. Once you start adding retarder thinner it not only makes it take longer to dry it makes it take longer to fully cure and harden properly.




What would cause that white haze when using a brush on lacquer?


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## Steve Neul (Sep 2, 2011)

Nick W. said:


> What would cause that white haze when using a brush on lacquer?
> 
> 
> Sent from my iPhone using Tapatalk


Whether you brush it or spray it the finish gets water in the finish and it dries so fast it skims over before the water can get out so it blushes. You could even apply lacquer in a space climate controlled and if you take it outside where the humidity is high it would haze a little. It wouldn't completely go white but you would see a difference in the sheen.


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## J_L (Apr 22, 2014)

Nick W. said:


> What would cause that white haze when using a brush on lacquer?
> 
> 
> Sent from my iPhone using Tapatalk


That's blushing. The finish is curing and trapping moisture in.


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## Nick W. (Jan 30, 2018)

J_L said:


> That's blushing. The finish is curing and trapping moisture in.




What gets the moisture out? Time? Should I put another coat on it? 


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## Steve Neul (Sep 2, 2011)

Nick W. said:


> What gets the moisture out? Time? Should I put another coat on it?
> 
> 
> Sent from my iPhone using Tapatalk


With a finish such as shellac or lacquer when you put another coat on it melts the dried finish so it has the potential of letting the water escape. This is why it's important to wait until a dry day to continue. If you get blush in two coats that is almost impossible to get rid of. What I have done before on lacquer if too much water has gotten in the finish is mix lacquer thinner and retarder thinner together and stand there and just spray coat after coat after coat until the blush clears up. The worst that can happen is you waste the thinner. 

On a finish such as polyurethane once the finish has skimed over if there is any water in it the game is over. Nothing left to do but to strip it off and start over.


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