On 24 May, 10:51, Cicero wrote:

(snip)

If you want to try using a chisel (say about 1/2") mark the position of the groove and score the edges deeply with a Stanley knife. Then work from the centre with your (very sharp) chisel towards the sides of the window reveal. Use the chisel almost flat with the surface. When you've created a groove clean it up by scraping the chisel (held vertically) along the bottom of the groove.

Thanks! I'm going to get a chisel from a local hardware store and do
this.