Automotive paints and relative humidity. Say wut?

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ACME SS

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I have a bad habit of pointing out the elephant in the room, you know the one thing nobody seems to talk about. Well... enter relative humidity to the reducer/activator speed equation. I have never seen humidity mentioned to any extent in a technical data sheet for automotive paint and I must ask the question, why not? I asked this of a paint manufacture and the response was, "interesting...we acknowledge that we could improve our TDS's", but it was left with that, just hanging in the atmosphere with neither confirmation that I was on to something or not.

This I know from experience:
-Paint a car in 100% humidity and you run the risk of blushing...solvent evaporation cools the surface and moisture forms, causing a cloudy film known as blushing. I experienced it once with lacquer and once with a urethane candy and in both cases a complete re-shoot was required.
-Hardeners/activators with isocyanates require a certain amount of humidity to work (I'm not a chemist but I believe they call it cross-linking and it increases viscosity which keeps the film from running and sagging but more important, it is the process by which the paint cures. Solvent based automotive paint cures this way, it does not dry per se.)

From the little I have been able to find on this subject (very little) it would seem that too low of humidity may hinder the cross-linking and cause runs despite having the "correct speed" reducer/activators and conversely, too high of humidity may cause blushing. To support the low humidity condition I did find an article from the uk where a manufacture was having problems in the winter with sagging (same temperature in booth) and the paint supplier told them to try to keep the relative humidity at 60%. There was no range given in the article...just 60% . That fixed the problem.

So my question is how does relative humidity play a role in the selection of reducer and activator speeds at a given temperature and humidity? Why do they not provide a chart that takes humidity into account?
Perhaps more simply asked, is there such a thing as too low or too high of relative humidity in which you should simply never attempt to paint in?
 
Interesting. I would love to hear, and learn, more about this. I plan on painting my car myself and relative humidity is quite low here in the dez most of the year (8-20%).
 
You have a good point ive been painting for 20 years. Ive never seen anything on humidity either. Id love to see the data on solvent vs waterborne also when it comes to the humidity
 
I just bought some primer and reducer from AUTO ZONE today and asked that very question ! Like some one at the Zone would know something. HA. But I have painted a few cars in my life , by no means a painter. Most turned out nice, last one I painted down here in FL in the heat and 98% humidity turned out rough with orange peel . Same with the primer I sprayed the other day. Rough and dry. Using regular Rust-Oleum reducer.
 
Yes they do make different speed reducers. But its temperature based not humidity
 
I have a bad habit of pointing out the elephant in the room, you know the one thing nobody seems to talk about. Well... enter relative humidity to the reducer/activator speed equation. I have never seen humidity mentioned to any extent in a technical data sheet for automotive paint and I must ask the question, why not? I asked this of a paint manufacture and the response was, "interesting...we acknowledge that we could improve our TDS's", but it was left with that, just hanging in the atmosphere with neither confirmation that I was on to something or not.

This I know from experience:
-Paint a car in 100% humidity and you run the risk of blushing...solvent evaporation cools the surface and moisture forms, causing a cloudy film known as blushing. I experienced it once with lacquer and once with a urethane candy and in both cases a complete re-shoot was required.
-Hardeners/activators with isocyanates require a certain amount of humidity to work (I'm not a chemist but I believe they call it cross-linking and it increases viscosity which keeps the film from running and sagging but more important, it is the process by which the paint cures. Solvent based automotive paint cures this way, it does not dry per se.)

From the little I have been able to find on this subject (very little) it would seem that too low of humidity may hinder the cross-linking and cause runs despite having the "correct speed" reducer/activators and conversely, too high of humidity may cause blushing. To support the low humidity condition I did find an article from the uk where a manufacture was having problems in the winter with sagging (same temperature in booth) and the paint supplier told them to try to keep the relative humidity at 60%. There was no range given in the article...just 60% . That fixed the problem.

So my question is how does relative humidity play a role in the selection of reducer and activator speeds at a given temperature and humidity? Why do they not provide a chart that takes humidity into account?
Perhaps more simply asked, is there such a thing as too low or too high of relative humidity in which you should simply never attempt to paint in?
If you keep asking questions like this you're gonna get banned from the forum for keeping us up at night, lol.
I'm sure when the humidity is up, like 60% or more, you're going to have problems. That being said, I've never thought about too low of humidity ever being a problem. This might be more of a question for a meteorologist. They might have an idea of how temperature and humidity work together or don't work together. Seems like a question that needs to be answered to give you the best chance of achieving a good paint job.
 
If you keep asking questions like this you're gonna get banned from the forum for keeping us up at night, lol.
I'm sure when the humidity is up, like 60% or more, you're going to have problems. That being said, I've never thought about too low of humidity ever being a problem. This might be more of a question for a meteorologist. They might have an idea of how temperature and humidity work together or don't work together. Seems like a question that needs to be answered to give you the best chance of achieving a good paint job.

Well what I find interesting is that you would think that high humidity would keep the solvent from evaporating as quickly and I am sure it does but conversely, the humidity is needed for the isocyanates to harden the paint, therefore it seems like something is missing in the technical data sheets (instructions). Isocyanate is basically super glue. If you were to put two dabs on something and put a drop of water on one and leave the other to "dry" on its own, the one with the water will probably get hot and crackle as it hardens almost instantly, whereas the air dry one will still be liquid. I have had some paint issues that could simply not be explained and I think humidity has something to do with it
 
I work in the aircraft industry and there is humidity requirements of when not to paint.
This is recorded inside the paint booths, even in a climate controlled building, the paint booths will go out when the sun starts to come up in the mornings.
 
Im assuming the blushing is just really bad solvent pop. Thats what we call it around here. Are you able to cut and buff it out or is it so bad it needs repainted?
 
When it comes to painting ive seen some weird crap. Had fisheyes for about 2 weeks. Our paint rep and painter at the time figured out my painter switched deodorants. These actually caused the issue. When he was in the booth sweating it was apparently getting air born. He switched back and it went away
 
Side note: when paint is engineered for application it is done in a temp and humidity controlled environment. I know here in the North East not everyone has a temp and humidity controlled down draft booth. Heated yes (most) but not like what they test the paints with at the manufacture. Maybe that is their “loophole” if stuff goes wrong.
 
Im assuming the blushing is just really bad solvent pop. Thats what we call it around here. Are you able to cut and buff it out or is it so bad it needs repainted?

It is not solvent pop. It makes the paint cloudy. Here is a picture of a car I did a few years ago. I could have let it dry and put a coat or two over it but it was a candy so the effect of seeing the base color would have been lost.
upload_2022-6-19_21-39-8.png
 
When it comes to painting ive seen some weird crap. Had fisheyes for about 2 weeks. Our paint rep and painter at the time figured out my painter switched deodorants. These actually caused the issue. When he was in the booth sweating it was apparently getting air born. He switched back and it went away

Very interesting!
 
Almost had a milky appearance. Ive seen it before. Ive never seen it in waterborne though. Definitely agree with you about the humidity and moisture being stuck in it
 
One of the best painters, hotrod builders I ever knew, years ago, liked to shoot when we were getting a little shower using acrylic enamel. He liked the slowest reducer possible also.
 
One of the best painters, hotrod builders I ever knew, years ago, liked to shoot when we were getting a little shower using acrylic enamel. He liked the slowest reducer possible also.
A little rain to keep the dust and temp down, and let it cure slow.
I not even sure you can buy acrylic enamel anymore.
 
A little rain to keep the dust and temp down, and let it cure slow.
I not even sure you can buy acrylic enamel anymore.

I use single stage acrylic urethane, but yes you can buy acrylic enamel.
The single stage stuff will last we after the clear is peeling off wet sand and buff that 20 year old single stage IF still enough, and see it shine some.
People (some) now are so anal they have to paint and clear the engine compartment, bottom side and even the friggin engine! But remem:poke:ber don't be pissed, I am old and senile!
 
I use single stage acrylic urethane, but yes you can buy acrylic enamel.
The single stage stuff will last we after the clear is peeling off wet sand and buff that 20 year old single stage IF still enough, and see it shine some.
People (some) now are so anal they have to paint and clear the engine compartment, bottom side and even the friggin engine! But remem:poke:ber don't be pissed, I am old and senile!
I agree with all you said. Clear coat is overrated for old cars.
 
I just bought some primer and reducer from AUTO ZONE today and asked that very question ! Like some one at the Zone would know something. HA. But I have painted a few cars in my life , by no means a painter. Most turned out nice, last one I painted down here in FL in the heat and 98% humidity turned out rough with orange peel . Same with the primer I sprayed the other day. Rough and dry. Using regular Rust-Oleum reducer.

The odds of running in to an employee at Autozone that has a clue what you are talking about is zilch I worked at 3 different stores in 5 years and I will tell you only about 10% of there employees would and almost 0 would have an answer for you I know enough about paint to know to call an expert and get an application sheet for the paint you are working with and follow it closely.
 
I use single stage acrylic urethane, but yes you can buy acrylic enamel.
The single stage stuff will last we after the clear is peeling off wet sand and buff that 20 year old single stage IF still enough, and see it shine some.
People (some) now are so anal they have to paint and clear the engine compartment, bottom side and even the friggin engine! But remem:poke:ber don't be pissed, I am old and senile!
Yeah my Willys I painted with single stage Urethane, but it will fade fast in the sun.
Quick buff and it comes back.
The old stuff had lead in it, 50 year old paint sometimes will buff back to like new.
Its what the EPA allows paint manufactures to produce.

Hey I cleared the engine compartment of my Duster, just because I never had an old mopar that shined under the hood, like the outside of the car.
opBPFj.jpg
 
A little rain to keep the dust and temp down, and let it cure slow.
I not even sure you can buy acrylic enamel anymore.

I used to think you couldn't and perhaps PPG and Martin Senour, etc do not but....my secret money saving supplier has it.
upload_2022-6-20_16-23-29.png
 
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