This week’s tech tip addresses a common problem in large-format polyurea applications: cold joints between spray passes, which can reduce the monolithic nature of the coating and create lines of potential weakness.
What Is a Cold Joint?
A cold joint occurs when the first pass of polyurea has cured enough that the subsequent pass no longer chemically bonds to it — the new material is bonding mechanically to a cured surface rather than co-reacting with partially-cured material. While polyurea-to-cured-polyurea adhesion is generally quite good (especially if the surface is still warm), the interpenetration that occurs when two passes bond while both are still reactive creates a genuinely monolithic film rather than a laminate.
Cold joints become a significant concern when:
- Application moves and returns to an area after the first pass has fully gelled
- Application is paused due to equipment issues, material changes, or breaks
- Section-by-section work patterns on large floors or infrastructure elements
The Solution: Window Management
The key is understanding your system’s intercoat adhesion window — the time after the first coat cures within which the second coat will still achieve inter-pass chemical bonding rather than purely mechanical adhesion.
For most pure polyurea systems at ambient temperatures, this window is approximately 15–60 minutes depending on formulation, temperature, and film thickness. Your material supplier’s technical data sheet should specify this window. If it doesn’t, call your technical rep — this is critical application data.
Planning Your Application Pattern
On large projects, plan your application sequence to work within the intercoat window. This might mean:
- Applying all passes to a defined section before moving to the next section
- Coordinating multiple applicators to maintain the intercoat window across the full working area
- Scheduling each pass sequence to complete before the first pass exceeds the window
For infrastructure rehabilitation applications like those discussed in our concrete rehabilitation guide, where multiple passes are standard for specified thickness, this planning is essential.
When You’ve Exceeded the Window
If a first pass has cured beyond the intercoat window, a light abrasive scuff (180-grit is typical) followed by solvent wipe restores mechanical adhesion and provides some surface activation for inter-pass bonding. Test adhesion with a pull-off test before proceeding if the delay was significant.
For secondary containment applications as covered in our secondary containment guide, post-application holiday testing will catch any pinhole defects at pass interfaces — a good reason to not skip that step even when you’re confident in your application.
Temperature Effects
Cold weather extends the intercoat window (slower cure), while heat shortens it. As discussed in our cold weather application guide, managing temperatures appropriately for your system chemistry is important for both film quality and practical scheduling.
The Weekly Tech Tip is published every Monday. Submit your technical questions to CPCA’s Technical Committee through the Contact page. See all previous tips in the Industry Resources archive.