Over the years I have collected a fair number of paint brushes, from cheap bristle brushes available from my local hardware store to expensive hog and even squirrel hair brushes that have been given to me as gifts.
As I paint in oil paints I have been regularly advised to avoid the fine nylon-hair brushes that are meant to be used for acrylic paints. I have found, though, that some of these are my favourite ones as they don't show brush marks on the canvas and they are better for blending.
The type of brush you use depends largely on what you are trying to achieve as a painter.
Opinions seem to differ as to the origins of the paintbrush but it appears that the sable brush was the best in older times and it is so today.
Artists' paintbrushes come in different shapes and sizes. There are generally four main types of brushes used in Oil painting.
The basic shapes are:
The FILBERT- a narrow flat brush with hairs that come to a rounded point.
The FLAT - a fairly wide but not thick brush with a straight edge.
The ROUND - the most traditional of the paint brushes- a thick brush that should come to a sharp point and hold a lot of paint.
The BRIGHT- a flat brush with short stiff bristles, good for driving paint into the weave of the canvas.
The other types of brush are the Fan, the Mop, the Angle and the Rigger.
I personally like the angle for making thin, straight line detail. I usually load up the brush quite heavily with paint and then scrape most of it off on the side of a container, leaving a fine, even spread of paint on the edge of the brush- when this is "dabbed" onto the canvas it makes a nice fine line.
All of the above brushes also come in different sizes ranging from a size 0 in some makes to a size 20. I have found though that sizes differ considerably with different makes, so that if you go into an art shop and ask for a size 8 brush, the salesperson will look at you strangely.
Generally, though it seems that the synthetic fibre brushes use different sizing methods from the "real" hair brushes. And, of course the "rarer" the hair used the more expensive the brush will be. The larger the size, the more expensive the brush will be, too.
It is a matter of taste and budget what type of brush you prefer. I have found that one of the cheapest brushes I bought lasted the longest and became one of my favourites. I couldn't bear to throw it out, rather like a favourite teddy bear, which is what it looked like when it eventually did meet the trashcan- there were just a few straggly paint-laden hairs left on the short stub of a handle.....sigh..
So there are really no hard and fast rules for oil-painting, though the purests would probably say that the bristle/hair brushes are the best and of these, sable is King!