When the ambient temperature has been cool, and then it shifts back to warm, the water jacket is going to act like a condenser and water is going to form from the humid air passing across it. The more air passing across it, the more water will form. Limiting the amount of humid air passing across it will reduce the amount of water formed. When the water jacket is warmer, and the air is cooler and dryer, the water will evaporate from it. It will requires less dry air to evaporate the water formed during the other phases. Using a water displacing coating, after cleaning the surface as thoroughly as possible, should prevent most rust as the water will be pushed off the surface, out of the crevices, and then sealed. A moisture collecting chemical drying agent will pull moisture from the air that manages to get into the firebox. After cleaning the firebox completely, might turn the circulator on and let the oil burner heat the firebox to 180 degrees, then coat it.