I wanted to give our fireplace frame and doors a makeover so I sanded and cleaned the brass, spray painted Rust-Oleum primer that is a gray color, and spray painted Rust-Oleum high heat paint that is black.

I have put a few coats on and it is uneven and some places. It almost looks like the gray primer is showing through and has a camouflage look. Any idea why it would not have a nice even coat?

Maybe give a light wet sand with finishing grade sandpaper.

High heat paint needs to be baked to fully cure. I don't believe I have ever seen a interior quality finish using high heat paint.
Might be better to powder coat.

We did that on ours and it looks fine. Used the matte finish, not gross.