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A Word About Surface Temperature
Air Spray Tech Consulting | Tech Corner
Don't Overlook the Temperature of the Surface You're Coating
When troubleshooting a finishing problem, we often look first at the spray gun, atomizing air, fluid pressure, coating viscosity, or application technique.
But there is another important variable that can easily be overlooked:
What is the actual temperature of the surface being coated?
Surface temperature can affect how a liquid coating wets the substrate, flows, levels, flashes, cures, and ultimately performs.
Surface Temperature Is Not the Same as Air Temperature
One common mistake is assuming that because the room or spray area is at an acceptable temperature, the part must be at the same temperature.
That may not be true.
Parts brought in from a cold warehouse, loading dock, truck, or outdoor storage area may remain considerably colder than the surrounding air for some time.
The opposite can also occur. Parts exposed to direct sunlight, ovens, heated production processes, or hot storage conditions may have a surface temperature well above the surrounding air temperature.
What matters to the coating is the temperature of the surface it actually contacts.
What Can Happen When the Surface Is Too Cold?
Applying a coating to a surface below the coating manufacturer's recommended application temperature can affect film formation and cure.
Depending on the coating system, low surface temperature may contribute to:
- Poor flow and leveling
- Extended flash or drying time
- Improper cure
- Reduced adhesion
- Condensation-related contamination
- Changes in final film appearance or performance
What Can Happen When the Surface Is Too Hot?
Excessive surface temperature can create a different set of problems.
Solvents or water may leave the coating too quickly, reducing the time available for the wet film to flow and level properly.
Depending on the coating and application, excessive surface temperature may contribute to:
- Dry spray
- Poor flow and leveling
- Orange peel or excessive surface texture
- Improper film formation
- Reduced wetting of the substrate
- Appearance or performance problems after cure
Don't Forget About Condensation and Dew Point
Surface temperature becomes especially important when cold parts are moved into a warmer, humid environment.
If the surface temperature is low enough, moisture from the surrounding air can condense on the part.
That moisture may not always be obvious to the operator, but coating over a damp or condensing surface can contribute to adhesion problems, film defects, corrosion, or coating failure.
For coatings with specific dew-point requirements, measure the surface temperature, ambient temperature, and relative humidity and follow the coating manufacturer's requirements before application.
There Is No Universal Temperature for Every Coating
A general temperature range can be useful as a starting point, but professional finishers should not assume that one temperature range applies to every coating.
Waterborne coatings, solvent-borne coatings, epoxies, urethanes, primers, stains, clear coats, protective coatings, and other finishing materials can have very different application and curing requirements.
Always check the coating manufacturer's current product data sheet or technical data sheet for the recommended substrate temperature, ambient conditions, humidity limits, and dew-point requirements.
Measure the Part, Not Just the Room
If surface temperature could be contributing to a finishing problem, don't guess.
Measure it.
The important question is not simply:
What is the temperature in the shop?
The better question is:
What is the temperature of the surface I'm about to coat?
The Bottom Line
Surface temperature is part of the finishing process.
Whether you're finishing cabinets and millwork, coating OEM products, refinishing vehicles, spraying fabricated metal, or applying industrial and protective coatings, the substrate needs to be within the conditions required by the coating manufacturer.
When a finish isn't behaving the way you expect, don't automatically blame the spray gun.
Check the coating. Check the environment. And check the temperature of the part.
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