Flow rate calculation of coating application

[Tom.G]

curing mechanism of the coating is a humidity reactive system.

Two possible ways I see this could cause your variations:

1) The spray nozzle is not being cleaned at end-of-shift and the residual compound is curing in it during the night.
2) Air used to pressureize the tank is moist causing partial curing. Air right out of a compressor is notoriously wet.

#1 has an obvious solution.
#2 As a test, get a tank of Dry Nitrogen with appropriate pressure regulator. Purge the reservoir,hose, nozzle, etc. and try that for a few days/weeks.

If #2 solves the problem, consider a dryer for the air line (expensive), perhaps a dessicant canister (periodic maintenance required) in the air line, or continue with the Dry Nitrogen tank.

Cheers,
Tom

p.s. As a last resort, farm out the coating process to someone else with a well-written/comprehensive quality requirement. The downside (beyond cost) is the incoming quality-control inspections needed.