In England I've seen such a thing for the first time and you can buy nice ready made dados by the metre in DIY stores. Not so in Germany.
From my German DIY store I got a pine board with the edges slightly rounded. I fixed it to the wall. Then I glued and nailed on a thick semi-circular pine slat and another one, this one being very narrow and attached with some distance.
I wanted the dado to look old and used. I gave it several layers of a light grey acrylic lacquer. To give it a dusty appearance I took some white matte wall paint and applied it thinly with a nearly dry brush so that long random streaks appeared. The raised sections I rubbed down slightly with fine steel wool.
Then I added some patina: I mixed a glaze from clear acrylic lacquer with a little burnt umber and applied this with a soft brush, immediately rubbing down the wet colour from the raised parts of the dado with a lint-free cloth. That makes the nooks and crannies look old and dirty.
Finally I darkened the glaze by adding some more burnt umber, took a toothbrush, dipped it slightly into the glaze and spattered the dado randomly to mimic those small dark spots flies leave on surfaces. The wall below the dado later was painted a simple light grey.
My dado now looks so old and dirty that people seeing it for the first time secretly (ha-ha) make the dust sample with their finger (the ladies!). Believe me, this is the most cleaned item in my home!
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