Evaporation rate is a balancing act
A sprayed lacquer coat needs its solvents to evaporate at a rate that's neither too fast nor too slow for the ambient conditions. Evaporating too quickly can prevent the coating from flowing out and levelling properly before it sets, and — in humid conditions — can trigger blushing (see the dedicated guide). Evaporating too slowly can leave the coating vulnerable to running or sagging, especially on vertical surfaces, and extends the time before the surface is safe to handle or recoat.
Working with the conditions on the day
Because ambient temperature and humidity shift, the same thinning approach that works well on a dry day may need adjusting on a humid or unusually warm one. Experienced finishers generally treat thinning and application technique as something to tune to the day's conditions rather than a fixed recipe.