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Should I use a primer under the high temp paint that will be applied to my L-Head cylinder head. Normall I use primer under all paint, but I don't know about the high temp stuff.

The original factory paint used a thinned layer of the outer 2430 OD as the primer. This was in a sense a self priming paint. If you want whatever high temp color you are using on your head to last any reasonable amount of time then follow the hi-temp paint manufacturer's directions for their paint.