Another thing that will make a difference in your finish is "overlap." Overlap is exactly that-the amount of area that is re-covered with each pass. You don't want to make each pass at the bottom (or top) of each existing pass. There should be a 50 percent overlap of each stroke. In other words, if your first pass lays down a 10-inch-wide pattern of paint, your next pass should overlap the first by 5 inches, your third pass should overlap the second by 5 inches again, and so on. This ensures that each pass actually gets two coats of material.
Triggering should begin just before the leading edge of the area is sprayed. Again, let's say you're spraying a door panel, for instance. Starting with the gun to the left of the door gap (at the back of the fender), you'll begin pulling the trigger slowly, allowing air to begin escaping from the gun. As you begin your stroke toward the door panel, you'll continue depressing the trigger until the fluid needle is actuated and fluid begins to exit the gun. By the time the gun is even with the door gap and at the leading edge of the door panel, the trigger should be completely depressed and fluid flow should be at its maximum. At this point you can continue with your stroke, depositing a full wet coat across the whole panel. As you reach the rear edge of the door panel (at the rear door gap) you'll release the trigger part way ("feathering"), until the flow of paint stops but air continues to exit the gun. You'll then drop the gun down so that the nozzle is even with the lower edge of your first pass (this will give you your 50 percent overlap) and repeat the action, this time moving from the rear of the panel forward toward the starting point of your first stroke. Repeat this action until the panel has been completely covered and you'll be ready to move on to the next panel.
I've also got a couple of points I'd like to make at this time (and these come from experience): First, when spraying any enamel or urethane enamel, I always make my first coat a rather light "tack" coat. I've found that by the time I've made a full circuit of whatever it is I'm painting that first light coat has "tacked up" enough to let a second heavier coat grab and hold on. It could just be my imagination, but it seems to allow the second heavier coat to "flow" a bit better without runs, sags, or drips.
The second thing I'd like to point out is that the selection of reducers and catalysts must be of the correct recommended type for the spray conditions, temperatures, and materials you'll be using. In this day and age of high-tech materials, a splash of this and a dash of that don't cut it! You've got to follow the mixing instructions for anything that's going to come out of your gun and onto the surface you've worked so hard to prepare. If not, you'll just have to prepare and shoot it again. I've learned that lesson the hard way!
Gun Control Next, I'd like to add two cents worth on trigger operation (or what's called "triggering" or "feathering"). The spray gun trigger operates in sort of a two-stage manner. As the trigger is pulled rearward, the initial movement actuates the gun's air valve, which allows air (and only air) to escape through the gun nozzle. As you continue depressing the trigger it then engages the gun's fluid needle, opening the fluid passage and allowing the paint to start atomizing and traveling to the surface. The farther rearward the trigger is pulled, the more fluid is released and transferred to the surface. You can compare the action to stepping on the gas pedal of your car-the harder you mash on it, the faster you go.
While we're talking about spray gun maintenance, this would probably be a good time to touch base on a few common spray gun problems you might run into-the most common it seems are funky spray patterns. If your spray gun exhibits a spray pattern that becomes much heavier at the top, bottom, or either side, you'll want to check to see if the air cap or fluid nozzle is causing the problem. You can do this by spraying a test pattern on a piece of scrap material. Then, rotate the air cap one-half of a turn in either direction. If the defect in the pattern is inverted, then the obstruction is in the air cap and it needs a good cleaning. If the defect in the spray pattern stays the same after rotating the air cap, then the problem is in the fluid tip. If so, the first thing to look for is a fine burr on the edge of the tip. If a burr is found, it can be removed by scuffing it with a piece of 600-grit wet or dry sandpaper. If there's no burr found, then you've most likely got a bit of dried material just inside the tip opening, and a good cleaning with solvent should solve the problem. An incorrectly adjusted fan control valve, too low of an air pressure setting, or too thick a material may cause an extremely heavy center in your spray pattern too.