The method
How paint-day works
The dew-point mistake
More than 40% of exterior paint failures in the first two years trace to wrong-weather application (PPG TB-100 / Sherwin-Williams). The most expensive and most-missed version is simple: a coat goes on during a fine afternoon, then overnight the surface cools below the dew point, moisture condenses on the curing film, and you get blistering, surfactant leaching, or peeling. The crew did everything right at 2pm — the failure was baked in by the forecast they never checked.
The rule we encode
The industry standard (ASTM D3276 / ISO 8502-42) is to keep the surface temperature at least 5°F above the dew point through application and cure. That 5°F margin exists precisely to absorb forecast and instrument error. We compute the gap for every forecast hour using the colder of the air temperature and the forecast ground-surface temperature, and flag the overnight window when it tightens — especially under clear skies and low wind, when radiative cooling pulls surfaces well below air temperature.
What we can and can't tell you
Forecast variables (temperature, dew point, humidity, precipitation, cloud, wind) are reliable 1–3 days out and decent to about five. The one thing we cannot source is the exact surface temperature of your specific substrate at the moment you coat — a sunlit south wall behaves nothing like shaded soil. So this is a scheduling and risk advisory, not a measurement device. We alert on risk, never on “go,” show you the raw numbers and the reasoning, and prompt you to confirm on-site with a $30 IR thermometer. You make the final call.
Data & sources
- Forecasts: hourly data from Open-Meteo.
- Thresholds: ASTM D3276, ISO 8502-42, and manufacturer technical data sheets.