compressed air and a very long air duster
Leaf blower with a narrow outlet in a pinch
Most decks really do not benefit from deck scrubbing
If the grass builds up there in use, it will do it the next time and the next time till eventually it gets to a point that the air passage prevents any more build up
However the top is what really needs to be kept clean because boy scouts start fires using dry grass & friction from rubbing sticks together .
Mower owners start fires from friction created by belts running in dry grass
Get at least 2 in every season.