You cannot see off-ratio in the spray fan. That is the whole difficulty. The gun sounds normal, the pattern looks normal, the material lands and sets, and a week later the surface is tacky, or soft, or brittle, and nobody can say when it started.
Plural-component equipment exists to deliver two chemicals in a fixed ratio, usually 1:1 by volume, and to mix them under enough pressure that the mixing is complete in the fraction of a second before the reaction takes over. When any part of that stops being true, the film chemistry changes, and it changes before anybody notices.
What off-ratio actually does to the film
The two sides are formulated to consume each other stoichiometrically. Shift the ratio and something is left over.
Resin-rich leaves unreacted amine in the film. The result is typically soft, tacky, under-strength material that may never fully harden. It stays that way; it is not a curing delay you can wait out.
Isocyanate-rich leaves free isocyanate looking for something to react with, and what it finds is atmospheric moisture. That produces carbon dioxide, which gives you foaming, pinholes, and a brittle film. It also means unreacted isocyanate is present at the surface for longer, which is a health and safety consideration and not merely a quality one.
Either way the material is not what the data sheet describes, and there is no repair short of removal.
Where the drift comes from
Temperature imbalance
The two components rarely have identical viscosity, which is why rigs preheat both sides and heat the hose. If one side is running cold, it is more viscous, it moves less freely, and the pump delivers less of it per stroke. The ratio drifts even though the machine is nominally set correctly. Cold mornings, a hose heater that has failed in one zone, a drum that came off a cold truck — all of these do it. Check that both A and B are at the temperatures the TDS calls for, at the gun, not just at the machine display.
Pressure imbalance
Watch both gauges, not one. A persistent gap between A and B pressure is the classic early symptom of trouble on one side: a clogged inlet screen, a failing pump seal, a partially closed valve, a restriction in the line. Small imbalances at rest can become large imbalances under flow.
Worn pump seals and check valves
Seals wear gradually. A pump that is slipping slightly on one side still builds pressure and still sprays; it just delivers less volume per cycle than the other side. This is the failure mode that produces slow, invisible drift over weeks, and it is why hour-based maintenance intervals exist.
Material starvation
A drum running low, a plugged suction screen, or a transfer pump not keeping up will cavitate one side. Watch drum levels; do not let a shift run past the bottom of a drum unattended.
Moisture and contamination in the A side
Isocyanate reacts with atmospheric moisture and will crystallize and crust in an open or poorly sealed drum. That crust plugs screens and damages seals. Keep the A side blanketed with dry air or nitrogen per the manufacturer’s instruction, and keep it closed.
Catching it before the tank is ruined
Run a ratio check. Dispense each component separately into clean, graduated containers over a fixed number of cycles and compare the volumes. This is the only direct measurement of ratio there is. Do it at startup on any job of consequence, after any maintenance, and any time the material looks or feels different. It takes ten minutes.
Log your numbers. Record A and B temperatures and pressures at startup and periodically through the shift. The value of a log is not the individual reading, it is the trend — a pressure differential that has been quietly widening over five jobs is visible in a log and invisible in memory.
Spray a test card, every time. A card sprayed at startup and set aside gives you a physical sample of what the machine was producing at a known moment. Check it for tack, hardness, and pinholing before committing to the substrate. On a job with any liability attached, keep the cards.
Believe the crew. Applicators develop a feel for how a rig sounds and how the material behaves under the gun. When someone says it feels wrong, stop and check the ratio rather than finishing the wall.
The economics
A ratio check costs ten minutes. Removing off-ratio polyurea from a containment basin costs days, and it costs them at your expense, because the material is going to be tested and the test is going to show what happened. The maintenance schedule in the equipment manual is not conservative. It is the cheapest part of the job.
General trade information, not a specification. Ratios, temperatures, pressures, and maintenance intervals are equipment- and product-specific — follow your equipment manual and the material TDS, and consult the SDS for isocyanate exposure controls.