Bare metals, do not 'wet sand'.
The minerals in the water will rust/oxidize the surface faster. That means you'll have to sand the rust away repetitively. Dry sand only or use alcohol or mineral spirits as a wetting agent.
100 grit is pretty course. Unfortunately you'll have to work up to ~500 grit before you can use any type of rubbing compounds to smooth it further W/O the use of paper.
If you know what kind of finish you're looking for I can assist you further. Steel is very easy to mirror polish or get most any finish in-between.