As for masking parts to be powder coated...you can use about any product, as the coating really is powder. I used Blue painters tape, so it wouldn't leave a glue residue. Then my powder coater removed the tape after coating/before the oven. Make sure your powder coater masks threads, as all don't. If not I found the best thing is to re-chase with a tap. You can get smooth or hammered surface textures, as well as gloss or satin, but I'm not aware of a flat powder coating yet. The baking process would make it difficult to produce flat I'd imagine. Smooth surfaces generally are about 2 mil, with textured about twice that. At least in my experience. Clear coating screw heads is an excellent idea.
I used to have a lot of pipe powder coated, for deck railings in Alaska. And when building custom computer cases.