Re: The filbert brush
I use synthetic brushes almost exclusively for my work. Early on, I noticed that my flats somehow seemed to become more useful as they became older, and I realized that it was because the corners of the flat brush were becoming less pronounced, and more rounded like a filbert.
I therefore concluded that I should buy filberts new, rather than waiting for my flats to become so used as to have rounded corners, so lately I've been doing that.
Although I find large flats be be very useful for roughing in my underpaintings, and for painting large, flat areas such as skies and water, I use my filberts for nearly everything else.
Clouds, flower petals, large leaves, the facial features of portraits, are some of the uses to which I put my filbert brushes.
I paint details with various sizes of rounds.