Scrim can be used on internal corners to prevent the plaster from cracking but external corners require special attention due to the fact that they often receive the occasional  knock or bump. In older properties it is not unusual to find wooden beading running down the corners of external corners on chimney breasts etc. Today either galvanised metal corner beading or plastic corner beading is used. This should also be used to bead reveals around windows etc. as these are areas which are prone to the occasional knock!