First, check the thermostat. Suspend it in a pan of hot water on the stove. A tall pan, dowel rod and string usually works. It also helps if the thermometer is suspended, too. Heat until the thermostat opens, I would bet you have one that is too hot.
Also, regular spray paint inhibits heat transfer. Eastwood sells radiator paint. Bad part is you have to boil the tank to get the paint off.
