groove

Hi, I am looking for external rendering on the front face of my house. Specifically, I want simple horizontal ashlar lines. The area is approx 20m and I'm looking to hire ASAP to beat the cold weather. Here is a rough idea of what I'm trying to achieve. Please get in touch.

I am trying to squeeze an antique desk into an existing alcove and frustratingly, the plastered walls are just a few mm narrower than the desktop. I would therefore like to find a way to create a shallow (3-5mm) groove in the plaster along the length of the wall, which would allow me to slide...