I learned the hard way! One coating that works is Zinc rich
galvanize paint used by electricians,if you want something that can
take any heat,but must be touched up now and then. A cheap hi-temp
paint is a waste of time! Will burn off and rust even if you
sandblast the pipes. CRC makes a good one,the same company that
makes the penetrating oil. I never used the one that Bill
mentioned,but I'm sure its good! I like the galvanize spray because
it is cured by heat and you can spray layer upon layerif it shows
rust!