I’ve used the Wagner Deck Mate for years. Its smart design really speeds up staining the planks in a deck. It has an aluminum handle, gravity-feed reservoir, control valve knob, and swiveling paint pad head. The long handle lets you stain the deck while standing up (and off your knees!)
Fill the reservoir with up to a ½ gallon of stain or sealant, and then twist the yellow knob to control the flow of stain down into the paint pad. As you glide the pad along the decking it both applies stain and back brushes for even and full coverage in one step.
A really clever feature is the center brush: a brush-within-a-brush that protrudes down and rides along inside the space between the decking slats. The stiff bristles both keep the paint pad in position as you go and get stain down along the side edges of the decking. Genius!
To clean, just place the Deck Mate in the sink and run water thru it.