For the frames coating, yes they are originally coated by their manufacturer. However with time, the coating gets deteriorated and we need to repair this coating defect with reference to standards and guidelines to avoide motors overheating due to improper application of coating in the field.
Unless operating conditions are already pushing the motor to its thermal limit, a coat of paint will not make a noticeable difference in the life of the motor.
Just my opinion....
This table is based on solar radiation absorption factors
White lead paint coating 0.25*
Light cream paint coating 0.35
Aluminum paint coating 0.55
Gray paint coating 0.75
Mat black paint coating 0.97.
*Not available for environmental reasons, for comparison only.