You have a couple problems.
1) You'll never get a high temp coating to stick to a header by "sanding" it. Parts need to be thermally degreased then profiled / blasted with a clean media. Then coat and cure as directed.
2) I don't believe you're supposed to prime and coat. Its either or, not both. I can't see one silicone coating sticking to another.
I've seen some BBQ / header paints work with limited success. Even less in turbo applications. I suspect after a few more attempts it's prob wise for your well being to get it ceramic coated and forget about it.
