Whatever type heat you are using....rule of thumb says that the metal you are painting should be the same temperature as the air.
If metal is too cold, paint won't adhere and just run down. I have a hot air furnace in my shop and I don't run it until I am in the shop. So if I go to paint something like a small bracket I will heat it with my heat gun to take the chill from the metal.
For doing big stuff , it's best to run the heat at a low temp for a good day or so to get all the metals warmed up.