I am working on a 54" mower deck for my JD-X595 right now. Cleaning the underside of caked-on dirt/grass from last year. When the unit was new, I applied a Slip-Plate type coating -- it was graphite-loaded acrylic. It covered and adhered very well, and I think it helped minimize the grass and dirt build-up over the cutting season.
But, one needs to keep realistic expectations for this kind of treatment -- the deck's underside is bombarded by everything that you run over. Stones, branches, dirt, gravel, mice, snakes, etc -- all gets picked up and propelled against the areas of the deck that you want the most protection on. So the coating gets worn off from where you need it the most.
Then you get to clean and recoat all over again. The option is to let the crud build up and reconfigure the aerodynamics of the deck underside, which screws up the cutting efficiency and explusion of the clippings.
Why isn't anything easy, at least permanent?
jim