"But there hasn't been much posted on the subject of interior heat coatings for the exhaust system." 

 

Can't hurt, reduces the heat flow to the metal slightly, but only slightly.   Too thin and has too high a thermal conductivity to  help much.  But if it makes you feel good.  Leave the outside surface alone and let it turn its natural black.  When the exhaust manifold has reached the end of its life due to thinning of walls extensive cracking, or creep (see picture posted by Brent), replace it.  Think of exhaust manifolds like oil filters.  Plan on regular replacement depending on the operating regime.  More demanding (such as turboappliction), shorter replacement interval. 

 

Don't even think about Inconel (very high temp alloy) exhaust headers.  Not worth it. 

 

"I might go with a mild steel system if I were to do it again."

 

Don't do it.  It will droop off due to gravity having absolutely no creep strength, and will drop in a heap in a few hours.  And it will rust to dust in a few cycles.  That is why stainless steel is used - because of its higher strength at elevated temperatures and because of its corrosion resistance under the same conditions.  Mild steel has no capability in this operating regime.

Fred

 

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