there's really no point in trying to de-rust the insise of the pipes, and any of the "rust-preventer" treatments aren't going to last because of the high heat (they are made for clean metal applications, mostly. your pipes go through such extreme heating/cooling cycles that rust is going to form on them/in them no matter what you do, and continue to catalyze. I think, if anything, the carbon buildup in the pipes is probably the best thing for preventing rust inside the pipes.