Just got a used HKS catback.

I have 2 questions.

1. I want to sandblast it and coat it with some rust proof stuff. I have some spray, but it goes on a borwnish/orange color. I've heard that some of those come in black. Anything you guys can recommend?

2. I noticed that the inside of the exhuast has a lot of carbon buildup, is it worth cleaning that stuff out, or should I just install it as is?