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Airbrushing & Conditions


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Being new to airbrushing I have a lot of questions as you might expect. The most nagging one is temperature and humidity. I have sprayed Tamiya Acrylic X-22 Clear thinned 1:1 with Tamiya Lacquer Thinner Retarder type in what I believe were near perfect conditions 70˚ with 50% humidity (measured with a Thermometer/Hygrometer). The stuff went down very well.

On the other hand, recently I tried spraying Tamiya X-18 Semi Gloss Black in 65˚ with 66% humidity in my basement (air conditioner had been running non stop for days) and no matter how I thinned it, 1:1, 2:1 with regular Tamiya Lacquer Thinner or their Retarder type, it would not go on even. It would dry quicker in some areas and slower in others. I tried applying mist coats as well as heavy with similar results. What did seem to help 'a little' was warming the paint and the part to be sprayed.

I hear people say they would never paint during days with high humidity while others say it can actually help slow down the drying time... but don't spray gloss paint.

Any insight would be appreciated.

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I won't paint unless the humidity is below 70%.  I airbrushed black acrylic yesterday when the humidity was around 67% and it worked out great.  Today I'm spraying lacquer gloss clear and the humidity is 55% and temp is 78 degrees.  I think the heat is less of a factor than the humidity. 

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I airbrush enamels thinned with lacquer thinner and I've never noticed a huge difference in ambient conditions. My normal painting garb is shorts and T-shirt and bare feet and if I'm not comfortable in my basement wearing that--too cold or too hot/sweaty--I just don't paint. 

Rattlecans are another story. Temp and especially humidity are a big deal there. The worst IME is Duplicolor. And almost any kind of clear gloss. 

Edited by Snake45
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3 hours ago, Snake45 said:

I airbrush enamels thinned with lacquer thinner and I've never noticed a huge difference in ambient conditions. My normal painting garb is shorts and T-shirt and bare feet and if I'm not comfortable in my basement wearing that--too cold or too hot/sweaty--I just don't paint. 

Rattlecans are another story. Temp and especially humidity are a big deal there. The worst IME is Duplicolor. And almost any kind of clear gloss. 

Here is NC Duplicolor is a gamble every day. Pretty much have to let it dry in the sun to avoid fogging.

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The dew point is actually a better indicator. Most people consider a dew point less than 60 degrees to be comfortable. Anything over 65 gets a little sticky because there is more moisture in the air. More moisture is your enemy and one of the causes of blushing, a milky haze on the paint surface. 

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1 hour ago, Rick L said:

The dew point is actually a better indicator. Most people consider a dew point less than 60 degrees to be comfortable. Anything over 65 gets a little sticky because there is more moisture in the air. More moisture is your enemy and one of the causes of blushing, a milky haze on the paint surface. 

Interesting view on the subject of humidity.   As a ground instructor for pilots, it is my job to explain this to aviators of all ilks.  Dew point is the point at which the air is at 100% humidity or the maximum water vapor content of the air.  The difference between the current temperature and the dew point is relative humidity.  At any given point the relative humidity and the difference between the current temperature and the dew point represent the  same amount of moisture in the air.  Pilots need this information to gauge cloud ceilings and potential for fog.  A difference between current temperature and dew point of 0, with calm winds will guarantee fog.   Having said that a 10 degree spread in dew point and current temperature is equal to about 70% relative humidity.   Two different ways to measure the same thing. 

Edited by Pete J.
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22 minutes ago, Pete J. said:

Interesting view on the subject of humidity.   As a ground instructor for pilots, it is my job to explain this to aviators of all ilks.  Dew point is the point at which the air is at 100% humidity or the maximum water vapor content of the air.  The difference between the current temperature and the dew point is relative humidity.  At any given point the relative humidity and the difference between the current temperature and the dew point represent the  same amount of moisture in the air.  Pilots need this information to gauge cloud ceilings and potential for fog.  A difference between current temperature and dew point of 0, with calm winds will guarantee fog.   Having said that a 10 degree spread in dew point and current temperature is equal to about 70% relative humidity.   Two different ways to measure the same thing. 

I have no plan on flying any time soon so I’ll stick to my general rule when painting. Thanks for the info though.

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Dew point is literally related to moisture content and the degree of temp where saturation occurs, thus at night temps fall below dew point and you find everything outside all wet,your lawn, car, barbecue grill etc. To me it's important to painting vs relative humidity . But it's just the gauge I prefer to go by, doesn't mean anyone else has too.

That said I've seen fogging of lacquers in cool temps too from spray cans. With low dew points sometimes, because the exiting of paint from spray cans chills it further to below the dew point and water gets into the picture in the form of that fog on the painted surface. Thus the heating of the can is two fold, it sprays better for one thing but raises that temp to where it won't drop below the dew point on the surface you're painting.

Airbrushing flat acrylic Tamiya and getting a rough finish is another matter. The uneven drying times over the surface of the model yet another and for flats actually fairly typical. Try the retarder lacquer thinner, 1 part paint to 1.25 or so thinner and see what you get. And you can push the drying with a hair dryer on the low heat setting if you want. That's my first suggestion. You might be better off with a dehumidifier in the basement, let the temp rise a little and still get the humidity a bit lower. Be nice to see your number reverse, 66 deg and 60 humidity. But I'm not convinced that's your issue.

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Looks like my comments were not very clear.  Dew point is not a static number when it comes to humidity.  Saying you won't paint when the dew point is above 60 makes no sense. That is like saying I have 6 oz of water, ask " How full is the glass?".  You don't know unless you know the size of the glass,  the temperature where your are painting is the size of the glass.  The dew point is the amount of water you have. 

  What matters is the spread between the dew point and the temperature.  A 15 degree spread will give you about 60% relative humidity and that would give you a good number.  However, consider this.  When the weather service gives you a dew point, it is for the conditions at the weather reporting station.  If it is hot outside and you are in cool basement or an airconditioned room the humidity will much higher and your paint is more likely to blush.  

  You are dealing with to many variables to refer to dew point as an absolute.  I have always had a hygrometer where I paint and I get a humidity reading that is reflective of my location.  I don't paint when the humidity gets about 70% or higher.  Hygrometers are cheap($10 to $20) and generally accurate enough for painting, but most important, they reflect the humidity in the room you are painting in and not the weather at the airport.

61WsTVg9YDL._AC_SL1500_.jpg

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1 hour ago, Pete J. said:

Looks like my comments were not very clear.  Dew point is not a static number when it comes to humidity.  Saying you won't paint when the dew point is above 60 makes no sense. That is like saying I have 6 oz of water, ask " How full is the glass?".  You don't know unless you know the size of the glass,  the temperature where your are painting is the size of the glass.  The dew point is the amount of water you have. 

  What matters is the spread between the dew point and the temperature.  A 15 degree spread will give you about 60% relative humidity and that would give you a good number.  However, consider this.  When the weather service gives you a dew point, it is for the conditions at the weather reporting station.  If it is hot outside and you are in cool basement or an airconditioned room the humidity will much higher and your paint is more likely to blush.  

  You are dealing with to many variables to refer to dew point as an absolute.  I have always had a hygrometer where I paint and I get a humidity reading that is reflective of my location.  I don't paint when the humidity gets about 70% or higher.  Hygrometers are cheap($10 to $20) and generally accurate enough for painting, but most important, they reflect the humidity in the room you are painting in and not the weather at the airport.

61WsTVg9YDL._AC_SL1500_.jpg

I agree with you Pete so take me off your list of ones not agreeing, at least as it applies to painting..

Edited by Dave G.
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Hmm, much to ponder. AIr conditioner hasn’t been running as much the past couple days and it is definitely more comfortable in the basement (shorts & t-shirt). Hoping to spray some primer today, though perhaps that’s not as good an indicator as actual paint.  I’ll post my findings later.

Thanks for the replies.

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On 9/3/2021 at 2:09 PM, Pete J. said:

Looks like my comments were not very clear.  Dew point is not a static number when it comes to humidity.  Saying you won't paint when the dew point is above 60 makes no sense. That is like saying I have 6 oz of water, ask " How full is the glass?".  You don't know unless you know the size of the glass,  the temperature where your are painting is the size of the glass.  The dew point is the amount of water you have. 

  What matters is the spread between the dew point and the temperature.  A 15 degree spread will give you about 60% relative humidity and that would give you a good number.  However, consider this.  When the weather service gives you a dew point, it is for the conditions at the weather reporting station.  If it is hot outside and you are in cool basement or an airconditioned room the humidity will much higher and your paint is more likely to blush.  

  You are dealing with to many variables to refer to dew point as an absolute.  I have always had a hygrometer where I paint and I get a humidity reading that is reflective of my location.  I don't paint when the humidity gets about 70% or higher.  Hygrometers are cheap($10 to $20) and generally accurate enough for painting, but most important, they reflect the humidity in the room you are painting in and not the weather at the airport.

61WsTVg9YDL._AC_SL1500_.jpg

Blushing is my biggest problem with painting, I need one of these.

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The ambient air's relative humidity (dew point) is very important factor while spray painting.  The dew point value is the air temperature at which dew will form on any object that is at that temperature or lower.  How many times you come out in the evening or night where you see your car (or other outdoor objects)  covered with dew?  How about when your cold beer, or soft drink glass/bottle/can gets covered with dew or "sweats"? That happens because those objects have temperature colder than the dew point of the ambient air.  The water molecules that are in the ambient air, when in contact with the cold object turn into liquid water and form dew on those objects.  That is as simple explanation as I can come up with.

How does that apply to spray-painting (airbrush or aerosol cans)?  When a compressed gas (propellant), depressurizes (leaves the nozzle), it cools.  When you use a spray can, you can feel it getting cool during the painting session.  This is due to the Joule Thompson Effect   . When that cooled paint/propellant mixture hits the model's surface and its temperature is below the dew point, it will form dew on the surface of the paint, causing "blushing".  It is as simple as that.

EDIT: One more thing that I forgot to mention is the type of paint (or specifically the solvent or thinner) used also makes a difference.  Evaporation of the solvent (once the paint is deposited on the model's surface) also causes cooling.  The faster the evaporation, the more intense cooling takes place.  Hot paints (usually lacquers) that dry fast, have fast evaporating solvents, and cause the most intense cooling.   That evaporative cooling of the solvent adds to the moisture forming on the wet paint's surface.

That is why moist (humid) ambient air is an enemy of glossy paint jobs.

Edited by peteski
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Update: Sprayed decanted Tamiya Grey primer thinned 1:1 with Tamiya's regular lacquer thinner. The conditions were 69˚ with 58% humidity. Air conditioner was not running today. No problems to report. Primer may have dried a little quick, however I think that might be due to the thinner.

I'll see if I can spray some actual paint tomorrow and report back.

Edited by 70 Sting
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On 9/3/2021 at 2:09 PM, Pete J. said:

Looks like my comments were not very clear.  Dew point is not a static number when it comes to humidity.  Saying you won't paint when the dew point is above 60 makes no sense. That is like saying I have 6 oz of water, ask " How full is the glass?".  You don't know unless you know the size of the glass,  the temperature where your are painting is the size of the glass.  The dew point is the amount of water you have. 

  What matters is the spread between the dew point and the temperature.  A 15 degree spread will give you about 60% relative humidity and that would give you a good number.  However, consider this.  When the weather service gives you a dew point, it is for the conditions at the weather reporting station.  If it is hot outside and you are in cool basement or an airconditioned room the humidity will much higher and your paint is more likely to blush.  

  You are dealing with to many variables to refer to dew point as an absolute.  I have always had a hygrometer where I paint and I get a humidity reading that is reflective of my location.  I don't paint when the humidity gets about 70% or higher.  Hygrometers are cheap($10 to $20) and generally accurate enough for painting, but most important, they reflect the humidity in the room you are painting in and not the weather at the airport.

61WsTVg9YDL._AC_SL1500_.jpg

Every a/c system I know about takes humidity OUT of the air thus helping to cool it. So how would an airconditioned room  be higher in humidity? I paint in a cool basement ( I think low humidity- gotta buy one of  those temp/ hygrometer things), central a/c in the house but the outlets in the basement closed. I still get die back with my gloss jobs.

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I painted my 56 body outside in the sun.  The temp was 79 and the humidity was around 67%.  Two days later I shot it with clear lacquer and the temp was around 72 and the humidity was about 62%.  Seem to work ok.  I'm still struggling with painting and figuring out the right temp and humidity.  I'm getting better at it.  Trying to get a few bodies painted for the winter builds.  

20210904_152238_resized.jpg

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The dew point is the key (the temperature at which dew forms from the ambient air).  See https://www.calculator.net/dew-point-calculator.html

In both instances the dew point was in high 50s - low 60s.  But as I mentioned, there are other things to consider (like how fast the paint's solvent evaporates, and that varies with the paint types and brands). The solvent evaporation rate is also directly dependent on the ambient air temperature -- that throws yet another wrench in the works. I don't think there is any specific magic formula (other than spraying when the dew point is in the high 50s or lower (which means the air is dry). 

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Update #2: Today the temperature was 70˚ with 55% humidity (basement). I sprayed Tamiya Semi Gloss Black, Gloss Black and Metallic Brown. All 3 were thinned with either Tamiya Lacquer Thinner Retarder type or Mr. Color Levelling Thinner. No problems to report.

The only thing I can conclude is that perhaps the cooler temperature combined with the higher humidity was the culprit.

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Phew, you guys sure have my old head swimming with all this.  My rule of thumb, and I paint in my garage, is humidity in the 60's and temps around 85 and below in the garage. Can't say my paint jobs are substantially affected tho. BTW, I use Createx and craft paint.    Interesting but somewhat confusing info....at least for me.

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1 hour ago, TransAmMike said:

Phew, you guys sure have my old head swimming with all this.  My rule of thumb, and I paint in my garage, is humidity in the 60's and temps around 85 and below in the garage. Can't say my paint jobs are substantially affected tho. BTW, I use Createx and craft paint.    Interesting but somewhat confusing info....at least for me.

Createx and craft paint are water-based paints. IMO, water (dew) will not affect them since water is the paint's solvent anyway. You can safely ignore all the confusing info.  Happy airbrushing!  :)

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If it is hot and dry outside I will shoot rattle cans. Inside the house with my paint booth, I wait until the humidity is below 50%. I also often run an electric space heater for a coupe of hours to get the room temp above 75F. Warming the cans in warm water is helpful for both paint flowout and pressure. I am still hit or miss with my airbrush using decanted paint regardless of the air quality.

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13 hours ago, ZTony8 said:

Every a/c system I know about takes humidity OUT of the air thus helping to cool it. So how would an airconditioned room  be higher in humidity? I paint in a cool basement ( I think low humidity- gotta buy one of  those temp/ hygrometer things), central a/c in the house but the outlets in the basement closed. I still get die back with my gloss jobs.

The ac unit does remove some moisture but it certainly doesn't take it to zero.  It only puts come cool air in the room.  A large portion of the air does not go over the coils.  That air is just the air that was there to begin with but with a lower temperature.    Cooling the ambient air, doesn't add water to it, but it does raise the relative humidity.  Regardless of what you may believe, get a hygrometer and check what the relative humidity is in the room you are in.  Hard numbers are always better than speculation.  

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1 hour ago, Pete J. said:

The ac unit does remove some moisture but it certainly doesn't take it to zero.  It only puts come cool air in the room.  A large portion of the air does not go over the coils.  That air is just the air that was there to begin with but with a lower temperature.    Cooling the ambient air, doesn't add water to it, but it does raise the relative humidity.  Regardless of what you may believe, get a hygrometer and check what the relative humidity is in the room you are in.  Hard numbers are always better than speculation.  

I have to disagree with you Pete about just "some moisture".  Any air-conditioning unit (windows or central AC at home, or a car AC) remove quite a bit of moisture from the air.  That is why the older windows AC units dripped water on the outside, and a car AC drips water onto the street.  Newer window AC units don't drip because the moisture condensed on the evaporator side gets splashed on the condenser side (evaporative cooling) to make the AC units more efficient. 

Further proof is that dehumidifiers are basically same construction as a window AC unit, except the evaporator and condenser coils are placed back-to-back.  When the ambient air is first forced to travel through the cold evaporator, the moisture condenses on it. Then the cooled (but dry) air travels through the warm condenser coils, warming it to slightly above room temperature.   While you are correct that AC does not remove all the moisture, it removes much more than just some moisture.

I have a window AC in my bedroom and also a hygrometer.  In my area summers can get quite muggy (dew point in the mid-70s).  When the AC compressor cycles on and off, I can see the swing in relative humidity.  Of course for the AC to remove moisture out of the air, that moisture has to be there in the first place.  In a desert climate, where the air is hot and dry, the dew point is low, and there is very little moisture to be removed.  So in that climate, AC does mostly just cool the air.  But in areas where the ambient air is hot and very humid (like the summers where I live), the dew point is high, and AC sucks a lot of moisture from the air.

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