If you are plastering i would use a plaster stop bead which is like a bead but only for one side, i'd fix with either nails or dryline adhesive depending what you are fixing to, then leave to set and plaster to that. If rendering or dashing you can get a similar stop bead but i prefer to use wood as posted above so that it doesn't rust, if you do use a bead when the render has set smear it clear silicone down the thick edge (the edge that will take all the weather as this will help protect it from rust, only silicone the bead edge though
thanks for replies but stop end for rendering to deep,i am plastering an inside wall a archway but only one side needs doing i cant nail batton onto other side will cause damage to wall,also if used batton surely when dry an remove it will crack n breakoff plaster,any other suggestions getting straight edge on only one side of external corner with plaster finish without causing cracking away.thanks again
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