I found out many years ago a liberal coating with decent waterproof grease on the whole inside of the inner wheel surfaces pays dividends in the long run...
It soon becomes covered in dust and grime from long journeys and helps form a protective layer....the outside I spray with corrosion guard on a regular basis after wash down.
If I take a wheel off for maintenance and scrape away the grease/crud it's like new underneath
I also coat the suspension arms ect and spray wax oil inside the Trailer frame...All best done from new,but worth doing anyway IMO...Because..
It works