You can apply a stain or finish to a large area faster with a rag or spray gun than you can with a brush. Rags are very effective when you intend to wipe off all the excess, as you usually do with stains and oil finishes. Spray guns are effective for applying any stain or finish and either wiping off the excess or leaving a finish to build a film.
In comparison, brushes are inefficient tools, because they can carry only a small amount of the liquid stain or finish at a time. You have to constantly dip the brush back into the liquid to reload it.
In cases where you aren’t wiping off the excess, finish leveling is very important. You can always sand the finish to remove flaws, such as brush marks and orange peel, and make it level. But this requires extra work, and it always carries with it the risk of cutting through and causing damage that’s difficult to repair. The goal in applying any finish is to get it as level as possible from the start. The three tools differ in their ability to produce a level film.
It’s nearly impossible to apply a film-building finish with a rag without leaving deep ridges in the film. (The exception is when you use the technique of French polishing, which can only be done with shellac.) Brushes perform better, but they leave brush marks. This is true even with sponge brushes, which leave ridges at the edges of each brush stroke, and with expensive bristle brushes. Spray guns produce the most level surface of the three tools. But even spray guns leave a light pimply texture called “orange peel.”
So, it’s virtually impossible to apply a perfect finish using any tool unless you wipe off all the excess after each coat. All three tools leave flaws in the finish film. To make a finish perfect, you have to level it using sandpaper, then rub it to the sheen you want using steel wool or rubbing compounds. It usually requires considerably less work to sand a finish level that’s been applied with a spray gun than one that’s been applied with a brush.
Waste is undesirable because of cost and the amount of pollution it creates. Waste costs more, not only because of the unused finish material, most of which winds up in the atmosphere, on the floor, or on spray-booth filters, but also because of the cost of disposing of used solvents and filters.
Rags and brushes are almost 100 percent efficient. By comparison, spray guns are very inefficient. A large part of the finish material is lost in overspray and bounce-back. HVLP spray guns are more efficient than the old conventional spray guns, and HVLP guns dominate the market because of this.