Ask the Expert Question-and-Answer Archive

by Ron Joseph

June, 2005

Humidity and Paint Performance

Q. Our company has recently purhased an automated paint system and have been debating whether or not to purchase a humidification system for the building it resides in. Next to the paint cell we have a wood manufacturing cell where we have been seeing warp issues due to parts drying out. The RH has varied from 22% to 65% this year (Minneapolis, MN). I was wondering how humidity effects spray painting and if there would be a benefit in regulating the humidity in the building.

A. Humidity definitely plays a role during the curing of waterborne coatings. You do not want the relative humidity to be too high because it affects the speed at which the water and coalescing solvents evaporate relative to each other.

I have never seen data on the effect of RH on the evaporation of solvents from solvent based coatings, but you must avoid the condensation of moisture vapor on the uncured film. Therefore, you must always insure that while the solvents are evaporating (which is a cooling process), you do not allow the surface temperature at the air/substrate interface to fall below the dew point.

Unfortunately, I don't have actual data in terms of temperature vs. humidity to serve as a guide, but in my opinion 22% RH is within the acceptable range. 65% RH might be somewhat high and you might want to bring that down to 50%. Don't forget that ambient temperature is also important and you should be considering both temperature and humidity. Without performing experiments I can't help you further.

If you would like me to research this further by going to the literature and the major coating research institutes I will be happy to do so, but I will need to do so on a consulting basis. Please get back to me if you are interested.

Best wishes,

Ron Joseph