Doesn't anyone clean their deck after every mowing like I do? I always clean the underside of my deck with a hard plastic scraper when ever I finished mowing. The accumulated green grass comes right off. Never left any buildup to mold or harden. No other special treatment. 30+ years and still using the same mower deck with only surface rust on the underside. And the non-greaseable sealed bearings are still original on my 46" deck on my JD318. Of course it's always been kept under roof in my pole building when not in use.
Interesting. I've NEVER cleaned under a mower deck. I owned 2 or 3 push mowers over 30 years for my smaller lawns until each wore out. Engines wore out, and cheaper to buy new than buy engines. None had any rust to my knowledge.
I bought a John Deere (sold under Scotts name) 12 years ago. I decided last year to get a new John Deere (now X500). The Scotts 54" deck had no rust, and very little dried grass accumulation when he picked it up. I use mulching blades, and only mow when the grass is dry. A long rain spell can cause me to have to mow twice due to grass height, but mulching blades clog up trying to mulch wet grass.
Maybe I've just been lucky. I take the deck off annually for maintenance (sharpen blades etc.) and have never seen anything other than a minor layer of grass sometimes, that if I worry about, I scrape off with a paint scraper.
I did try to use the cleanout hose attachment on the x500 once. It looked like the water was cleaning the heck out under the deck for about a square foot immediately around the attachment and that was it. I called my John Deere salesman, who confirmed that was a marketing gimmick, and knew of know one who thought it was of any value.
I'm not disagreeing with any of the previous posters who need to do the extra work to keep their deck clean. Perhaps its the grass type here (mainly tall fescue) or maybe its when I hit the few areas that suffer a sparse layer of grass - the deck gets a quick sandblasting.