Explain how humidity affects solvent evaporation.

Question

What will be an ideal response?

Answer 1

Though humidity does not affect the drying and curing time as much as temperature does, it is still a factor, and should be taken into consideration when painting.
When the day's humidity goes up, the amount of vapors that can be held in the air decreases; therefore, solvent does not escape as quickly as it should from the finish that has just been sprayed. If a day has 40% humidity, the solvents will evaporate much faster than on a day with 90% humidity, when the air around the vehicle being sprayed is already saturated with vapors.
Though most technicians make their choice of solvent solely on the temperature, the humidity should also be taken into consideration. If a day's temperature is 85° F at 90% humidity, it may be wise to choose a solvent that evaporates faster than temperature requires, to compensate for the high humidity of the day. So, if the technician has the choice of 70° or 85° or 95° solvents, he should remember that the warmer the day, the faster the solvent will evaporate, so slower evaporating solvents are formulated for hot days. If a technician needs to compensate for the humidity, he should choose a slower evaporating reducer, or one temperature lower. In this case, the best choice would be 70° reducer.