What percent humidity "should" produce a blush free clear coat? I'm using Rustoleum 2X clear. Or, for the pro's, what humidity level do you require before you paint? I put a de-humidifier in the room set at 45%.

Hi Perry,

Rustoleum is fine at 45% humidity, and it is much less sensitive to humidity level than is a fast evaporating material such as dope.

Don't forget to factor in the temperature as I have had problems with low humidity and high temps.

Randolph says 50% max if you care about blush.
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Randolph makes enamel, dope, and polyurethane paints.  Each product has its own sensitivity to humidity with dope being the most sensitive to humidity, enamels next less sensitive, and 2-part polyurethanes being the least sensitive.  I spray dope in a temperature range of about 65 to 95 degrees F, and in a humidity range of about 65 to maybe 70% maximum.  I will spray in low humidity such as 10% RH and temperature in the range shown above and add butyrate retarder as needed.  This is for full-scale airplanes as well as models.  You just have to try spraying & adjust the mix & technique as you go.

Enamels can be adjusted somewhat by varying the speed of the reducer, and for acrylic enamels a hardener is available that improves the gloss & chemical resistance appreciably.  Two-part polyurethanes are the easiest to work with in widely varying temperature/humidity conditions; various speed reducers are available along with various speed catalysts as well as accelerators to accelerate the cure time.

Rustoleum (spray cans) is an excellent product but there is not anything you can do to vary the paint mix.  You just have to pick your conditions and you can end up with excellent results.     

We tend to focus on the humidity reading but ignore the dew point.  While watching the weather reports here in DFW for the past few weeks, the relative humidity some days was around 50%, temperatures of 90 F or above, but the dew point was around 72 degrees.  Maybe these conditions are OK for finishing systems other than dope but with a dew point that high I would never attempt spraying dope.

Hi Jim, I agree about the dewpoint factor.  I overlooked it because here in So Cal it is very rarely a factor.  Good point.

What percent humidity "should" produce a blush free clear coat? I'm using Rustoleum 2X clear. Or, for the pro's, what humidity level do you require before you paint? I put a de-humidifier in the room set at 45%.

Perry,

I've never paid any attention to it. I said many times I paint and clear all my models outside paying absolutely no attention to weather conditions except for rain.

I've applied the clear I use on really damp days and on days with great sunshine without ever having an issue with blushing. One day I had just made it before it started to rain.

I don't use dope or Rust-o-leum.

The only blushing I experience is with myself when I get kudos for the finish on my models. 

Here's one example.

CB