This is another segment of a general discussion of spray gun technique. Whether you are a first timer or a seasoned finisher, there are items of interest in this series that I encourage you to read. Please refer to past week’s articles where I discussed transfer efficiency.
Overlap is a very important part of laying down a consistent coating. Overlap goes hand in hand with the size and shape of a correctly adjusted spray pattern. Without defining that size and shape, one is only guessing about how much overlap from one pass to the next is necessary.
A correctly adjusted spray pattern is a very elongated oval shape. Around the perimeter, there is not as much coating as in the center. However, from end to end, the center is both straight and even. If it’s not, then gun cleaning and adjustment are necessary. Evenness and straightness of the oval are essential.
To check the pattern shape, hold the gun very steady in your hand, trigger the gun only far enough to make air come out of the air cap. Do not pull the trigger far enough to release fluid…yet. You will feel resistance in the trigger when it comes up against the fluid needle. Now, take the gun and point it at a piece of scrap material, cardboard or heavy paper taped to the wall. Hold the gun off of that surface at the distance that you feel is appropriate for optimum transfer efficiency. Quickly and smoothly pull the fluid valve all the way open and then release it as quickly as you opened it. A quick burst of coating should appear. The shape of that pattern should be as described above. If it’s not to your liking, feel free to experiment with the atomization air pressure, the fluid control valve, and the fan control valve until you have what you want.
Again, that having been said, if the pattern isn’t right, take the gun apart, clean it, and start again. It makes no sense to spray with a funky pattern. Make another pattern test after each adjustment so that you can keep track of your progress and the direction that you are going. Work at it until you get the shape that you want.
Footnote. I always have a large sheet of brown paper or cardboard taped to the booth wall. I use that to test my gun. When it’s used up, I throw it away and put up a new one.
That process completed, it is time to do some more experimentation. Again, use a scrap, a piece of cardboard or heavy paper. Spray a pass. Then, come back to the beginning point and start the second pass with the center of the pattern on the edge of the previous pattern. The idea here will be to make a 50% overlap between passes.
Move the gun smoothly across the test piece working to keep the distance between the spray tip and the piece always the same. At the same time, move the gun across the test piece at a constant rate. Try to be as robot-like as you can in your approach to laying the coating down on the test piece. Don’t be afraid to add in more fluid or trim the volume of fluid back if necessary so that you get the correct film thickness for the speed you are willing to go.
As a footnote, if you are interested in knowing how much wet film you are laying down during this process, may I suggest that you find a wet mil gauge and check that?
I wish that I could tell you that everything you spray will be just as easy as spraying that test piece. But, I’d be lying. From here on, the application of the coating to your project gets tricky and requires some serious thought.
Come the end of the day, what you want to have accomplished is to have the same wet millage of coating applied everywhere on the object you are spraying. Assuming that you are working as robot-like as I have suggested above, then the coating should very evenly applied. But, is it?
Let’s say that you are spraying a flat panel; perhaps the simplest item to spray. What will be the overlap for that first pass?
a. Will the top edge of your pattern be at the edge of the panel?
b. Will the middle of your pattern be at the edge of the panel?
c. Where will that pattern hit the panel to insure consistent mil thickness all across?
The answer is B.
Like a roofer shingling a roof, that first layer has to look like all the others. Lift that bottom shingle and you will see a starter course underneath. You need to lay down that half pass in order for the next pass to have its proper foundation and to achieve it proper millage at the edge of the panel. Any other option will leave you scant at the edge.
The same is true of the opposite edge of the panel. How you exit or finish up your spray pattern is as important as how you begin it.
I do the same thing when I mow my lawn. The grass abuts the concrete or the flower beds. I make my first pass all the way around with the middle of the mower on the edge of the grass. That way I can make my second pass with both the mower’s wheels on the grass. And off I go!