Hooker is correct in saying that if you only drive your car to the local cruise in, or grocery store, it WILL rot off. That is the WORST thing you can do to your exhaust. For every gallon of gas you burn, you produce one gallon of water, along with multiple corrosive particles. If you don;t drive the car everyday, that water vapor and corrosive contaminates stay festering in your pipes. If you drive it every day the heat will burn the nasty stuff out. If you run race gas in your car and only for short trips, a mild steel system will rot off in about a week.
Race gas does TERRIBLE things to steel. If you are going to store your car for any long period, run unleaded fuel through it for about an hour of so.
I had to recently throw a real nice set of ported iron heads away becasue of race gas corrosion. I first tried to sand the ports clean. Came back out into the shop the next day, and there were rust "spots" in the exhaust ports. I then took some carbide to the ports and removed about .015" off of the port walls (exhaust oly) and the next day the same thing happened. I ground some more, and it came back! (kinda like a cat you really HATE) I ended up having to grind so much, that the heads were JUNK! Into the recycle bin they went. Wasted a set of $1,200 cylinder heads. The valves were fine, so I saved about $350 on my next set. (though, I do all my own porting, but it cost me more than 25 hours of dirty/hard work.
) These heads had been run ONLY on C-16, and were pulled off to rebuild the engine. They sat for about 2 yrs in open dry air. (well, as dry as it is in my shop, that never drops below 50 degrees) The corrosion "worm holed" into the water jackets. It WILL do the same to your exhaust system.
Stainess steel, whether its 409, 304, 321, or Inconel, will out live the rest of the car. Even inconel will change to a brown color when it reaches 1000+ degrees. If you want to keep polishing your 304 tubing, it will look nice for a long time. Alot of work, but if that is what you like, have at it. A light rubbing with Scotch-Brite* will also keep it looking great.