I had a lawn tractor engine freeze up, and I pulled the head to fix the problem. While I had it apart, I put several parts into my ultrasonic parts washer to try to remove the caked on dry grease. It cleaned the parts really well, but apparently removed the surface coating of the metal that prevents rusting. Totally exposed, the parts began surface rusting in less than a day -- not good.
What can I coat the metal parts with to prevent surface rust that can handle the heat of a small engine? I've got clear electrical varnish, WD-40, petroleum, silicon lube, gun oil, etc. What works best? Any "don't use" protectants that would adversely impact the engine?
Bonus question: The ultra-sonic parts washer is a new acquisition. Are there things I shouldn't clean with it? It seems to remove essentially everything on the surface of metal, both the nasty stuff, and any protective coatings, paints, etc.