Every month, our instructors go through the Coatings Academy member forum and pull the questions that came up most often. This month’s Q&A features answers from Lead Instructor Carlos Mendez (Houston) and Director of Curriculum Rebecca Chen. Whether you’re a new student or an experienced applicator, we think you’ll find something valuable here.

Q: How humid is too humid to spray polyurea?

Carlos Mendez answers: This is probably the most common question I get in workshops. The short answer is that pure polyurea is remarkably tolerant of humidity—far more so than polyurethane. You can spray in conditions that would completely halt polyurethane work. However, there’s a practical limit.

Most pure aromatic polyurea systems can be applied in relative humidity up to 90–95% without significant issues. The key exception is substrate temperature relative to dew point—your surface temperature needs to be at least 5°F above the dew point temperature regardless of ambient humidity. A substrate below dew point will have surface condensation that compromises adhesion.

For hybrid polyurea/polyurethane systems, you need to be more careful—check the manufacturer’s specific humidity limitations. Some hybrids behave more like polyurethane and have lower humidity tolerances.

Q: I’m getting inconsistent color in my aromatic polyurea. What causes this?

Rebecca Chen answers: This is almost always an off-ratio condition or material temperature variation. Aromatic polyurea is not UV stable and will amber/yellow with UV exposure, but if you’re seeing inconsistent color immediately after application, look first at your proportioner.

An off-ratio condition where the B-side is slightly high relative to A-side will often produce areas with slightly different color as well as slightly different physical properties. Regular ratio verification—using test kits at the start of each job—is the professional standard for a reason.

Material temperature variation is the other common culprit. If your heated hose isn’t maintaining consistent temperature from the proportioner to the gun, you’ll get viscosity variations that affect atomization and can affect appearance. Check your hose temperature from end to end.

Q: What’s the right overlap technique for achieving consistent film thickness?

Carlos Mendez answers: We spend a lot of time on this in our workshops because it’s where a lot of new applicators struggle. The standard technique is a 50% overlap pass pattern—each pass overlaps the previous one by half the spray fan width. This produces even film thickness distribution when applied at consistent distance (typically 18–24 inches for most guns) and consistent speed.

The common mistake is varying your speed or distance. Slowing down at the edges of a wall section or speeding up in the middle produces high spots and low spots. Practice developing a consistent rhythm before you’re on a paid job.

For vertical surfaces, I teach students to apply in horizontal passes bottom-to-top, which prevents sag better than top-to-bottom application. The first pass is your thinnest tack coat; subsequent passes can be applied faster (thicker) once that tack coat provides some build.

Q: How do I approach bidding a large secondary containment project for the first time?

Rebecca Chen answers: Secondary containment is high-stakes work because the regulatory consequences of failure are severe—for the facility owner and potentially for you as the applicator. Here’s my advice:

First, make sure you actually have the credentials for the job. If the spec calls for a certified applicator, that means our Polyurea Applicator Certification or equivalent. If it calls for an inspector, that’s our Inspector Certification. Don’t bid on work you’re not credentialed for and learn on the job—the stakes are too high.

Second, do a thorough site visit before pricing. Secondary containment projects have high variability: penetration count, cove detail complexity, substrate condition, accessibility, and the specific chemical service. Each of these factors affects your labor estimate significantly.

Third, don’t try to lowball your first containment job to get it. Quality facilities are paying for compliance and performance—they’re not primarily price shopping. Price your work based on your actual costs and a fair margin, and win or lose on your credentials and approach, not your price.

Have a question for our instructors? Post it in the member forum—it may be featured in next month’s Q&A edition. Not yet a member? Enroll in any Coatings Academy course to get community access.