I don't know If I would use a fish tank heater. That requires motion to heat the tank properly. I wonder if adding an air pump might work? As long as you are at room temp it works fine. Someone mentioned using an ultrasonic cleaner to strip parts on here a long time ago, and some of those have heaters. Don't over think this to much. I do all the time and it just slows me down.
BTW If Steve says they look fine and paint away. Do just that, His work is awesome.