Tool or trick recommendations for cutting lines in render into an inside corner, one face only

[robinbanks]

Through experimentation it turns out the look most preferred for "lines in render to look like blocks" is to cut them afterwards rather than scratch them in as the render is going off - perhaps I bought the wrong weberpral, but the stuff I have has some fairly coarse aggregate (2mm) in and it tends to make the lines a bit of a mess if a lump of aggregate drags out as the line is being scored in the soft/going off render

So far using a grinder with a straightedge so it's something like a tracksaw is giving the desired look but it doesn't work very close to e.g. window frames or other things in inside corners that shouldn't be clipped with the cutting wheel.

Casting around for ideas as to how to cut nice lines into inside corners - is there perhaps some diamond bit (like one I saw for removing mortar for pointing but I think that was a bit too big) I can use in a router or drill? Or perhaps something I can use with a 90 degree adaper, or in a dremel?

At the moment I'm putting a part groove in the render after it's started to turn, and then cutting the rest of the line when it's gone properly hard. It works ok but I'll probably end up having to blend the look by repointing the "bed" later on because the grinder and the scratch don't look exactly the same

 

[mattylad]

Multitool.
buy one.they are cheap enough, get some spare blades when you