On Fri, 9 Nov 2018 15:55:53 +0000
Andrew wrote:

And filling long cracks like that without getting any on the beam is
easy if you use a plasters trowel instead of one of those silly
'filling' knifes.

Just use the filling knife to build a line of filler along the
long edge of a plasters trowel and then wipe it into the gap, away
from the beam.

No need for a knife, you just move the plaster around on its board or
hawk until you can pick up an even line along the edge of the trowel.