Hi 'Wota'

When you say outside, do you mean 'outside' as in exposed to the air and not inside? If so then ordinary plaster is NOT the thing to use, as plaster is an indoor material.

If it is outside, fill in the cracks as best as you can and skim over with cement. Do this to as many coats as it takes to level off.

You could then finish it off with a coat of outdoor or masonry paint, as cement wash on buildings get dirty after a year and looks bloody terrible [my own point of view].

Kirk