I tried a factory washout on an old mower once ! What a mess it produced, and didn,t come close to cleaning out the underside of the deck. Being retired, I have the luxury of being able to choose when I cut our couple acres of grass. Like others have stated, I only cut it when it is 100% dry. In the fall, once I have removed the mower from the tractor to mount the snowblower, I tip it up and give the bottom scrape with a 3 inch putty knife to remove any areas that have a little build up in the corners. At the same time, I have a good look at the blades, and touch them up if required, I also check the level of oil in the gearbox, and grease all the nipples and the sliding shaft, so it is ready to put back on in the spring.