TomZ is completely right..exhaust pipes rust from the inside mainly because of all the water that runs through them. If you want to put something cheap on the outside you could use any high heat paint such as the BBQ paint

 

i was thinking about it and it may sound stupid but the part that most rusted was the outlet pipes in the back of the muffler, i was thinking "can i paint the inside of that pipe before the bend?" that was the worse spot

 

lol go to an exhaust shop have them cut u enough pieces to fit your truck in a short enough length to coat the inside, then you can still weld the outside ha....

 

or i can just cut pipes 12inches long and have them go in to the muffler point down and just not worry about rust all together haha