Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2005

Why Does Paint Run on a Cold Surface (Substrate)?

Q. I was asked this question earlier this evening and was at a complete loss for an answer. Why does paint run much more easily on a cold substrate? It is something I've never done because I know what will happen but I've never known why.

A. Andrew, I really like your interesting question. I too have never given much thought to it, but my opinions are based on simple physics of liquids. Here's my attempt to answer your question:

Statement #1: When a paint or coating is cold it has a higher viscosity than when it is warm. It acts much the same as syrup that we pour our pancakes. Therefore, on a cold day it will have less of a tendency to run than on a warm day, when its viscosity is lower.

Statement #2: Regardless of the surface or coating temperature, when the solvents in the paint or coating evaporate from the coating its viscosity increases and it has less of a tendency to run. Therefore, when the paint is first applied it has more of a tendency to run than after a few minutes when the volatile solvent have evaporated.

Statement #3: When a paint or coating is applied to a cold substrate or surface, evaporation of the solvents will be slower, but the initial viscosity of the coating will be higher. Its tendency to run will be balanced between its higher initial viscosity (less ability to run) and the slower solvent evaporation rate (greater ability to run).

The reverse is true for warm substrates. The initial paint or coating viscosity will be lower (hence a greater ability to run), but the solvents will evaporate more quickly (less ability to run).

In my opinion it is not a forgone conclusion that a coating will have a higher tendency to run when applied to a cold substrate. Some coatings might well do so, but it will depend on their initial viscosity and the evaporation rate of its solvents. Since the formulation of coatings differ depending on how the resin and the solvent blend, I don't think we can come to a universal conclusion. I'm always open to hear another opinion and if you can tell me that your observation is always true, I'd like to here from you again.

The situation is even more complex when we consider this problem for waterborne coatings. Other factors come into play, and I won't even try to cover them here.

By the way, if this topic is of interest to you please visit www.ronjoseph.com/po.htm to see the class I teach on painting operations.

Best wishes,

Ron Joseph