1st thing you do with a lettering brush that you will use in oil based paint, the day you buy it.............you put in a tray that will allow the 10W MOTOR OIL to cover it completly. now leave it in that oil for at least 24 hours.
the reason for this is what other refer to as THE HEEL, of the brush, this is the point where all the hairs come together, then is inserted into the Ferrule. in most brushes its metal, plastic or some organice material. the metal is pinched to hold the hairs, the quill type is twist tied with copper wire. at that point is where paint will build up even when cleand properly. soaking the brush in 10W motor oil, will allow the oil to reside in the heel...........rather then paint. paint kills the brush when it builds up in the heel.
HAIR....is exactly what most good lettering brushed are. FRENCH SQUIRREL HAIR. the rodent is caught, the hairs are removed from the tail. reason the tail hairs...........because of their lenght, strength and color....GREY or BROWN. each has a quailty unto its own. browns tend to be longer lasting and withstand more use, greys are finer and softer for doing work without brush lines. back to the HAIR.....these hairs are removed from the squirrel by plucking them outa the skin. the skin end goes into the furrel, the other end of the hair is a perfect POINT!!! so when you put a group together, it will not leave brush marks, or "hollywoods" we call them, holes or hollows in the paint you just laid down. also they tend to flow paint better and cut clean edges.
the reaso you keep them in oil is they are a natural material...like your hair, without oil it dries out, cracks and will disintegrate with out oil.