I have had good luck with this method of cleaning mower decks. This clears a lot of grass etc so locate mower where a mess is OK. Get your garden hose with the water on (the larger the mower the more water). place the hose just outside of the blade housing. Start mower and activate the blade. The water floods the area under the mower and the blades pick up the water and more or less water blasts the underside.