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Author Topic: Tolerable humidity range for spraying dope and lacquers?  (Read 4188 times)

Offline Mike Scholtes

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Tolerable humidity range for spraying dope and lacquers?
« on: October 28, 2016, 02:37:50 PM »
I live in fairly dry Northern California, but we do sometimes have humid-ish days. I have seen posts from our friends in the Deep South that humidity can make painting essentially impossible. Is there a temperature and humidity range where spraying dope and lacquer paints is optimal, and a range where it is not prudent to even try?

Offline Jim Svitko

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Re: Tolerable humidity range for spraying dope and lacquers?
« Reply #1 on: October 28, 2016, 04:08:49 PM »
I think there is more to it than just the humidity.  Temperature and dew point also play a part.  You could have fairly low humidity but if the dew point is high enough you could get blushing as the solvents evaporate and cool the surface to the dew point.

I do not like to use much retarder so if the cheapie humidity gauge in my garage says 70% or more I do not paint.   If it is too hot, the dope dries so fast that orange peel is the result.  As far as dew point goes, I prefer to see it around 60 degrees or less.  I found that has worked well for me.  I am in Fort Worth where we can have high humidity as well as very dry days.  Usually the dry days are in the fall/winter when the cold fronts bring crisp, dry air, but some summer afternoons have humidity of 25%.  Of course, it is also about 105 degrees at that time.



Offline Steve Helmick

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Re: Tolerable humidity range for spraying dope and lacquers?
« Reply #2 on: October 28, 2016, 06:14:18 PM »
It's been awhile since I had a problem with blushing. I was probably using hardware store lacquer thinner, which is a bit of a risky business. But cheap. My recollection is that I solved the problem by turning on an electric space heater to warm up the shop...and then turning it off. I'd never use a propane or any other combustion type heater, since all produce moisture as a by-product.

The heat relates to the dew point, but was a gut feeling solution for me. I had to Google up a definition of "dew point", because it's been 50+ years since I took any science classes. Below is the definition I found, in case somebody would benefit.  D>K Steve


"The dew point of a system is the temperature where the amount of vapor inside a system becomes the saturated vapor. In other words, for a closed system, the dew point is the temperature at which the dew starts to form. At the dew point, the relative humidity becomes 100%. Any temperature above the dew point will have relative humidity lower than 100%, and any point below the dew point will have relative humidity of 100%. The dew point is a temperature and, therefore, it is measured in temperature units."
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Offline Mike Scholtes

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Re: Tolerable humidity range for spraying dope and lacquers?
« Reply #3 on: October 28, 2016, 07:57:01 PM »
Thanks everybody, and to the private message I also got. I always use Randolph's butyrate thinner for painting; the hardware store stuff is for cleaning brushes and nozzles. I have not actually had a problem, but the humidity today was 81% which seems like a lot to be doing any dope painting. I am aware of dew point issues as I am a full-scale pilot and that is life-or-death knowledge.


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