Having spread a lot of fertizer over the years I would like to offer a partial alternative to some of the above posts. Yes, for protecting solid panels of steel these are the right things to do with coatings and such. Most of the rust, however, won't form there.
The nitrogen-urea-fertilizer that we use and is the topic at hand is like sulfuric acid in it's ability to eat steel. I have found it is not so much the solid surfaces that are the problem but the connectors BETWEEN them and all the small parts associated with these low-priced spreaders.
The inability to coat all of these parts and the tendency of them to vibrate usually cleans them to bare steel shorty after use. Even if a coating would stick to them initially, it would vibrate off from sharp edge contact and the urea would certainally attack or wick under those connecting parts and surfaces without being wash removed and re-coated with something that would wick into the tiny surfaces to protect the clean bare steel. No one suggests a slathering of motor oil but an air-pressure cleaned, washed and air-pressure dried spreader sprayed with a small about of diesel fuel and oil (synth if you want) will do the job better than other methods I have seen. I've found it to be the only way.
Even the plastic/metal Vicon small parts will rot if not properly cleaned. The Vicon I had had a lot of carbon steel and castings blended in with the plastic and stainless.
Just my two cents here but coat away at the solid surfaces but spray some sort of wicking oil into the nooks and crannies after clean and dry would be my personal opinion.
