I have little faith in a deck wash access port that allows you to connect a garden hose to flush out the underside of the deck. I do not think they work as well as one would expect.
I would sooner drive the thing up on ramps, and use a hose and nozzle so I can see what's there, and what's getting removed. Just adding water to spinning blades won't guarantee a clean underside.
The best thing for longest deck life is to remove damp/wet clippings as soon as possible so the metal is not subject to long periods of being wet. Dry impeded corrosion as the 'electrons' can't travel as well over dry, so the oxidation takes longer.
No matter what coating is used, the grass & weeds in my case, will abrade the coating leaving relatively bare metal. Leave the grass on, and corrosion will occur.
I think it's a long-term battle, rather than a 'one shot' to keep corrosion limited.