If you want to try something different, paint the underside of your deck as usual, wait a day or two to dry, and then coat the entire underside with two-three coats of polyeuthane. After the poly is dry, apply two or three coats of car wax. Try this for one season, and see if it makes a difference.
As for washing your decks out each time you mow, make sure you run the deck for at least five minutes when you're done, so that you dry out the bearings and belts. If you don't, the water lays in the bearings and will rust, and wet belts will eventually dryrot and get brittle with humidity. If you have a blower, blow off the worst of the grass first before soaking the deck with water. Water spray will get most of the grass off, but it will also trap the grass.