depends on what you want to remove and how bad the stains/growth is you want to get rid of - also how long do you want to hang around seeing if its worked or need a second/third application...
Plastics 1 part hypo 3 parts water - light use
Medium stains - algae and lichen 50/50
Heavy growth of lichen - light covering of black spot 3 parts hypo 1 part water
Black spot (heavy). Neat
Darran