One of the most important decisions affecting the outcome of your project is choosing the right type of brush for the kind of paint you are using, the surface you are painting and the environment you are painting in.
If you are painting with an oil-based paint, a natural bristle paint brush (usually hog or ox hair) is the right choice. The reason is that these brushes are soft and supple and will deliver a finer, smoother finish than their synthetic counterparts. Truth be told, you can also use synthetic brushes with oil and brush manufacturers are now making synthetic bristles specifically for oil, but pros will always go for natural over synthetic. White bristles work best on smooth surfaces while black bristles are ideal for rougher textured surfaces because of their extra firmness and abrasion resistance. If you're refinishing furniture with a lacquer (for example) or otherwise require an ultra-fine finish, ox hair is the best choice. Natural bristle brushes should never be used with water-based paints as they will absorb moisture, lose their shape and become limp.
When painting with water-based coatings a good quality synthetic bristle brush is required as they don't absorb water and will keep their shape. These brushes are usually nylon, polyester, nylon/poly blend or a variety of other synthetic materials unique to various manufacturers. A 100% nylon brush will deliver paint to your wall or trim more smoothly but this material doesn't perform as well in hot or humid conditions. When you can't control the climate go for a nylon/poly blend, it's will hold up better. Most of today's generation of paints require a fairly stiff bristle, so don't equate a soft brush with a better finish when working with water-based paints.
Still unsure, give us a call and we can talk you through it.