I made my living for a 45 years as a professional sign painter/pinstriper, Gallery artist and a career in painting graphics on just about everything, including the use of an airbrush, not to mention the actual painting of cars, mostly classics and show cars and custom show cars.
I can honestly say, I've seen the quality of brushes go down hill since the middle to late 80's.
As far as a brush to shape and cut in letters, sign guys would use the "quill." I can remember purchasing ten and only having three or four that felt good and worked well enough the to do a lettering task or pictorial with confidence.
Because money meant nothing, that is, you would think an expensive brush would be of quality, but they may or may not be, I found myself purchasing ALL my needed brushes, actually to make my living with, from a company called Mack Brush. I'm sure they are still in business. Just Google.
Brush drag, is generally what causes the hairs to be pulled out of the ferrules. When paint is put on to dry or if it tacks up quickly, hairs will be pulled because the paint actually gets tacky or sticky. Sign guys would use different oils or transmission fluid to oil their brushes.
The oil would be applied and worked deep into the ferrule. Brushes would never be allowed to get dry. After use, they would be oiled again and left wet from the oil.
Brush drag again. Very common, even when using a guill for truck lettering that two coats were needed. White over a red truck.
Here's a variety of my work. The last photo is art in progress, I'm working on it,. No brushes here, I'm using a palette knife.