It is possible to achieve a very smooth finish using a spray gun. You should be warned, however, that there is more preparation and work involved when using a spray gun. The first is that the paint has to be a particular thickness and consistency for every brand of spray gun. You will have to discuss the matter with the personnel at the paint store to make sure you have the right combinations. When painting with a brush or roller, usually some newspaper or plastic sheeting on the floor is enough protection. With the gun, you are going to have to cover everything in the room. As careful as one might be with the gun, everything must be covered. If you do use the spray gun it is extremely important to use even strokes in alternating directions. When spraying, keep the gun aimed straight at the wall, and turn it off at the end of every stroke. Keep the gun the same distance from the surface at all times. If not, the paint will not be distributed evenly.