Adjusting Viscosity in Liquid Coatings

Mike Bonner of Saint Clair Systems discusses the problems that can arise from adding solvents to liquid coatings to adjust viscosity. 


Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon
Saint Clair Systems

Mike Bonner is the vice president of engineering and technology for Saint Clair Systems. 

Q: My paint supplier gets really edgy when we add solvent to our coatings to adjust viscosity. Since it all comes out in the oven, does it really matter?

A: Most coaters don’t think about the fact that adding solvent to reduce the viscosity of their coating is fundamentally changing the formulation of their paint.

But that is exactly what it is.

I’ve never understood why we expend so much effort to develop coating suppliers we can rely on, work diligently with them to develop a coating solution that perfectly fits our customer’s demanding specifications — the right color, gloss, adhesion, protection properties, etc. — and then, at the first change in the weather, take it upon ourselves to alter that formulation by adding solvent “so it will run right.” And to make matters worse, we compound the error by blaming them when this “new formulation” doesn’t meet the original specification!

It seems a little absurd when you think about it that way, doesn’t it?

So how can changing the formulation be an acceptable solution?

The answer is: It isn’t. In fact, it’s a form of surrender. It’s our way of admitting that there are factors in our carefully designed process over which we have no control and we just don’t know what to do about it.

It’s not that we don’t try to implement suitable controls in our process. For instance, we might:

  • Create a staging area to get the paint to the right temperature before we bring it to the line
  • Develop summer/winter blends
  • Install paint heaters


The biggest problem with staging is real estate. It takes a lot of floor space. And we’re going to expend energy (read: money) to keep this space at a constant temperature. To keep our footprint as small as possible, we might stack the paint from floor to ceiling. But heat rises, so the paint at the top will be warmer than the paint sitting on the floor. This can be addressed with sophisticated air circulation systems, but that, too, adds cost.

Staging is also logistically complex. We have to carefully schedule to assure that the right paint makes it to the staging area with enough time to acclimate before it goes to the line. Plus, there is excess handling required. Though a common approach, it’s not an elegant solution.

Summer/Winter Blends

This puts control over the formulation back in the hands of the experts — the chemists. But since no good deed goes unpunished, it comes with trade-offs. Now, you have to track two different part numbers for the same paint. This often increases the cost by reducing the volume of each part number. This can be a competitive problem because coating is a low-margin business. Once you get past the cost ramifications, you have to manage the volumes very carefully to make sure that your winter blend does not carry over into the summer months and vice-versa. This seems simple on an individual basis, but when applied to dozens or even hundreds of colors over the course of a year, it too gets to be a real logistical nightmare.

Paint Heaters

Another common approach is to try to manage the temperature of the paint once it reaches the process. But there are a lot of things to consider when you contemplate installing any kind of heating system into your coating process. As we discussed with staging, coating is a low-margin business so you may not have a lot of capital to play with — and these systems run from the tens-of-thousands to the hundreds-of-thousands of dollars — which ups the ante on this decision.

There’s also the issue of safety. Heating in an intrinsically safe environment can be tricky business — and a mistake here can be far more catastrophic than a few blisters!

Then there’s the question of what to do in the summer.

Heating only addresses the winter concerns. Conventional paint heaters can offer, at best, a 6-month solution. When temperatures climb in the summer, they simply can’t help. 

So, what is the solution?

Obviously, the key to controlling any process is to stabilize all of the variables, and with a coating process, formulation is key. Anything short of the optimal formulation is a compromise. Of course, the performance of that perfect coating formulation is integrally linked to temperature, so stabilizing the process means stabilizing the coating temperature at its optimal point — all-year-round.

It’s really that simple.