Plastering Q: When to use external angle trowel

[Finally_motivated]

I want to make good the external corners on a chimney breast (between the front and the two cheeks).
Currently, the bottom half of the corners seem to have been done by hand,ie there is no expanded metal angle beads. The top half of both corners still has the old wooden dowel in place. I do want to move the old wooden dowel.
I want to avoid hacking back at the corners and would prefer to use one of those external angle trowels. Can anyone offer any advice on using one of these and on my particular scenario in general.