I remove the decks from my mowers after the last cut and remove the blades, which are then sharpened. While they are off, I power wash the underside of the deck to remove all caked on grass.
When it dries, I use an old paint brush to liberally coat with used motor oil. Finally, I grease the fittings and install the sharp blades. The engine also gets an oil change and the chassis lubed at that time.
In the spring, everything is ready to go and rust on the deck has never been an issue. Odds are, if you leave that caked on grass mess on thru the winter, that is when most of your rust occurs. I learned that lesson the hard way, with my previous mower, which I serviced each spring and usually flaked off big chunks of rust and deck while scraping the grass off the underside.
I still applied the used oil when I did the service in the spring. Without having the whole winter to dry, a lot more grass would stick to it during the first few cuts. Fall service is definitely the way to go.
