The vast majority of hobbyist woodworkers, and quite a few professional woodworkers - who might only complete a project or two a month - do their finishing with brush or rag. However, as you begin to undertake more projects, or you move to larger carcass work, it's natural to start thinking about spray finishing.
There are at least three good reasons for considering a switch to spray finishing. First, it's surprisingly easy to achieve a near perfect finish with spraying. With practice, you'll be able to apply the finish evenly and uniformly in a lot less time than you would by brush. Second, because a spray finish goes on thinner than a brushed finish, it dries fairly quickly. Dust doesn't have much time to contaminate the surface, and you can apply any follow-up coats that much sooner. Third, because spray finishes are fast and easy to apply, you can spend more time building projects rather than finishing them.
A spray finishing system consists of two devices that work in tandem - a continuous source of pressurized air, and a gun that atomizes the finish and delivers it in a controlled pattern onto the work surface. Since the advent of spray finishing, air compressors have provided the necessary source of pressurized air. The compressor forces air at high pressure into the spray gun, which atomizes the finish and projects it at high pressure (upwards of 60 or 70 PSI) onto the work surface. The drawback is that, because of the high pressure, only about 30 percent of the finish lands on the work surface. Most of the finish either bounces back from the surface or ends up sprayed around the work surface. Quite a bit of that wasted finish material ends up vented into the atmosphere.
High Volume Low Pressure (HVLP) systems significantly reduce the problem of over spray and bounce back. They rely on a high volume of air at a low pressure (a maximum of 10 PSI) to transfer upwards of 80 percent of the finish onto the work surface, which is better for your pocket book, and the environment. And, because the air is travelling at a low velocity it's a lot easier to control the spray.
There are two types of HVLP systems. If you have an air compressor you can purchase an HVLP spray gun that runs off the compressor (these are referred to as 'conversion spray guns'). Because these spray guns require a lot of air you'll want to use a compressor in the 5 HP, 60-gallon range, and with an 80 percent or higher duty cycle, otherwise the compressor is going to by constantly cycling on and off.
If you don't have a compressor, consider purchasing a turbine HVLP. The turbine is somewhat like a mini compressor, except that it delivers a consistent high volume of air at low pressure to the spray gun. Turbines also warm and dry the pressurized air, which helps the applied finish cure more quickly. As well, turbines are very small, so they can be easily stored or moved from one location to another. You'll find that turbines are rated by the number of fans (called 'stages') they contain. The more stages, the greater the volume of air and pressure the turbine can deliver, and the wider the viscosity range of the finish you can use without having to thin the finish.
Ready-to-spray water based finishes are ideal for use with HVLP systems in small shops because, unlike solvent based finishes, they are non-flammable and non-combustible, and they contain fewer environmentally hazardous materials. As well, they're practically odourless. These finishes have great clarity as they don't have the typical amber cast that you get with solvent based finishes. And, cleaning up your gear at the end of the finishing session is relatively easy. You do, though, need to pay somewhat more attention to temperature and humidity with water based finishes than you would with solvent based finishes.
Even though water based finishes might be considered 'friendly' finishes, you'll still need to use some kind of spray booth, provide adequate ventilation, and wear an appropriate respirator rated for organic compounds. For small shops, a free-standing knock-down booth is a good choice for spraying large projects. Duct tape pieces of heavy cardboard (appliance boxes work well) or coreplast together for the body of the booth. This makes it easy to quickly set up the booth when needed, and then fold it up and store it between uses. A small, bench top booth is handy when you want to spray smaller pieces. As an alternative to cardboard you could also use plastic sheeting. Set up the booth the night before so that dust has time to settle down before you being spraying. And, for the best results it's important to illuminate what you're spraying, so that you can differentiate between wet and dry areas. Instead of overhead lights, use free-standing lights that you can raise, lower, or angle to better illuminate the work. A simple shop made turntable is also invaluable for rotating your work so that you can spray all sides.
If you're at the point where finishing your projects with a brush is beginning to take too much shop time, then you really should think about spray finishing. A good place to start is one of the better books on the topic, such as "Spray Finishing Made Simple" by Jeff Jewitt, or "Spray Finishing" by Andy Charron. Once you've made the plunge, spend the time to get acquainted with the spray system, and practice until you're satisfied with your results before you commit to a first project. The time you invest in learning about the equipment and practicing with it, will pay huge dividends in the long term.