Hi.
The only thing I'd not use is underseal, it can get water trapped behind it and actually make things worse.
Make sure there is no rust initially and if so clean it off with a wire brush, then use Bilt Hamber De-oxit rust converter, either use their wax spray or Waxoyl. Keep the beam clean, even a wash with a garden hose is enough, make sure the holes in the cups are not blocked with grit or even wax as these need to drain.
After that a check each year and re-application of the wax if necessary will be fine.