Anything suitable, with a thin board of thickness to gauge how much it should be smoothed out in thickness. Screw one in at each end on the wall and place others in between to align in line with them. A string works well to use. Distance between arbitrary, when the plaster has set for a while, the boards are removed; a little tapping makes it release better, then the groove is plastered again. Long wall harder, short wall easier, best to plaster the entire wall. Plaster that is joined after it has set leaves a line.
To get it plumb, you might need to have some small pieces of wood behind the boards. The wall could be uneven, and the worst is if it leans in or out, then it can become thick on part of the wall. I had a slope in the cottage of 4 cm myself. It was a lot to smooth out, of course, you accept some slope in old houses. When you start, you dampen the wall, wait a while, and apply a thin layer of mortar, let it set. It creates a rough surface that the actual plaster adheres well to then.
I have personally used wooden strips, they are usually available in about 10mm thickness and are straight.nimhed said:
Ready-made sheet metal strips are available at building markets. They are plastered in and remain in place.
But they can't be scraped against, if they are to be plastered in they need to be lower than the surface of the plaster.H hempularen said:
Or are they separable so one part is removed and the other part remains in the plaster?
Technology advances, so maybe that variant exists.