When metal heats up, it can sweat, like a glass of cold beer taken out of the refrigerator. This is called dewpoint condensation. Obviously the adhesion of a coating is seriously impaired or non-existent. To prevent this from becoming an issue, surfaces should not be painted if they are within 3°C of the ‘dew point’, which will vary from metal to metal and can be determined using a psychrometer to determine the humidity, a thermometer to determine the temperature, and a chart such as below, or better still an online calculator.
https://www.lamtec.com/technical-bulletins/dew-point-table/
Briefly, the higher the humidity the higher the dewpoint, for example at a temperature of 21°C and humidity of 100% the surface with be wet at 21°C or less, whereas at the same temperature and 50% humidity the dewpoint will be 10°C, so the surface to be coated should be at least 13°C.
