To the best of my knowledge sterling has to be nickel plated prior to rhodium plating or the oxidation will show through. Here is a comment from Ganoksin that I found on Google. Worth the read. Apparently you can also use palladium as an underplating.
You are correct Janet that the rhodium solution will be contaminated
if you put the silver in there directly. The trade pre-plates the
silver with nickel or some other plating that rhodium will not be
contaminated with and will adhere well. Electrocleaning is a must in
all steps.
Russ