In this brush pen portrait from 2017 I used a large two-inch wide flat brush to lay in washes. Then a 0ne-inch wide flat for details. (You can turn a flat on its edge remember.)
It was great fun. Flat washes on a rounded set of surfaces.
I’d been playing with flat brushes before but by the end of 2017 I’d focused on them, and I still am focused on them. It’s rare I even pick up a round now, unless I’m out in the field and have to use a water brush.
I don’t have a favorite, I have a bunch I reach for. I have my filberts I use for gouache of course, but I’ve now got a collection of of long and short flats of all sorts of different widths. Princeton has a number of lines you can try.
Try mops, flats, brights, and long flats (called different things by different makers). Don’t forget filberts. I do so love filberts.
The important thing is to pick some up and start making some marks!
Here’s a close up.