In my experience the only exh manifold coating that really lasts is hot ali spraying, also known as hot zinc by others,
It literallly stays on for many years,
You could try other methods but you'll probabably need to do it several times,
I use RFI Screening in Braughing/herts, recent cost as an example was about £45 to sandblast and hot ali spray a full length crossflow manifold,
So budget on about £90 for a pair of v8 full length headers.