The underslung pipes are usually always rusted and pitted , but you can make yours slicker than new.
First clean and remove all rust. I like to use a coarse sanding sponge and some Krud Kutter. Then prime with rust arrester.
Then brush on a good heavy coat of Por 15 manifold coating, let dry and sand off as much as you can with medium paper.
This will almost level the surface. If not, repeat until you are satisfied with the finish. I used three coats of Por 15.
When you're happy with the finish, spray on a coat or two of high temp clear coat.
Works well on manifolds and mufflers too.
The Por 15 and high temp clear coat are available at most automotive paint stores.