De-trickulatatin a header coating conundrum.

No matter what you do and no matter what you use there is still the problem of heat expansion of metals!! When metal expands (under heat) it tends to sluff off anything that is attached to it simply because all materials expand at different rates (iron, cast iron, metal, aluminum, paint, etc). The coating you use will expand at it's own rate and separate itself from the base metal as the base metal expands at a different rate. You didn't do anything wrong but if the coating doesn't expand and contract at the same rate as the base metal there will be some separation. This is true of any paint system. That's why they make high heat paints!!
Adherence can also be a problem but regardless of how well a paint adheres to a surface, expansion and contraction can still separate the base metal from the coating. Surface preparation is also be very important but heat expansion can still separate the metal from the paint/primer if they don't expand and contract at the same rate.

My two cents!!

treblig