Site conditions
Recoat or drying time adjusted for temperature (10 °C rule)
- where
- factor applied to the data sheet time
- temperature the data sheet time refers to (°C)
- expected surface temperature while drying (°C)
- estimated recoat time (h)
- recoat time on the data sheet (h)
Explain this equation
Adjusts a data sheet drying or recoat time for the temperature you are actually working at, so you can plan coats on a cold or warm day.
tr is the recoat time printed on the data sheet, which applies at its reference temperature Tr, usually about 20 °C. T is the surface temperature you expect while the coat dries. f is how many times longer (or shorter) it will take, and t is the adjusted time.
Chemical and physical drying processes speed up with warmth; a common rule of thumb, Arrhenius in origin, is that rates double for each 10 °C rise. So the time is multiplied by 2 raised to the number of 10 °C steps colder than the reference.
Sense check: a 4 hour recoat at 20 °C becomes about 8 hours at 10 °C and about 5.7 hours at 15 °C. At 30 °C it would fall to 2 hours.
Watch out: water-based paints also depend heavily on humidity and air movement. Treat the result as a rough guide and test with a fingernail before recoating.
Scales a data sheet drying or recoat time to the temperature you are actually working in, using the rule of thumb that times halve for every 10 °C warmer and double for every 10 °C colder.
Enter your values to see the result.
Good to know
Chemical reactions roughly double in rate for each 10 °C rise (an Arrhenius-type rule of thumb); coatings suppliers use the same rule for cure speed, based on the substrate temperature rather than the air. Hempel notes that drying and overcoating times depend on temperature, humidity, ventilation and film thickness, and that solvent-borne pot life halves for each 10 °C rise. Data sheets usually quote times at about 20 °C; check which temperature your data sheet uses.
Common mistake
Assuming the data sheet recoat time holds on a cold, damp day. In winter a 4 hour recoat can easily become 8 hours or more, and recoating too soon traps moisture or solvent.
Limits
Rule of thumb only. Water-based paints dry mainly by evaporation, so humidity and air movement matter as much as temperature; high humidity can slow drying far more than this predicts. Never use it to justify painting below the product minimum temperature.
Sources
- BlastOne: Basic principles of accelerating cure time (rule of thumb: each 10 °C rise in substrate temperature doubles cure speed, each 10 °C drop halves it)
- Hempel: Explanatory notes to product data sheets (theoretical spreading rate = volume solids % × 10 / DFT µm; WFT = DFT × 100 / volume solids; volume solids to ISO 3233-1; drying and overcoating times depend on temperature)
- Dulux Trade Diamond Matt product data sheet (volume solids 49% white; minimum application temperature 8 °C per BS 6150; recoat 4–6 h; VOC max 1 g/L against 30 g/L limit, cat. A/a; thin up to 1 part water to 10 parts paint)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.