Turn the heaters off and allow them to cool.
Sand any remaining paint or rust from the hydronic baseboard heaters. Vacuum up the rust and paint and wipe down the heater with a cloth.
Lay a drop cloth under the heater and place tape on the wall around the heater to avoid paint messes. Open any windows in the room for ventilation.
Apply a primer to the baseboard heater with the paintbrush. Coat the entire heater and allow it to dry for 24 hours.
Dip the paintbrush into the paint and apply a layer of paint to the baseboard heaters. Get into every nook and cranny on the heaters tubes to cover it with paint. Allow the paint to dry for 24 hours. Apply a second layer after the first layer of paint is dry.
Allow the paint to dry for 24 hours before using the heater.