Proper spray gun setup and spray technique are critical to obtaining consistently good finish work. This starts with spray gun selection. Those of you still using old siphon cup guns should get rid of them. You should be using an HVLP gun; I prefer a gravity fed version. Not all gravity fed guns are HVLP, so check before you by. You should be able to get a decent quality gravity fed HVLP for about $150.
The next most important consideration is needle/tip size. This must be matched to the material you are going to be spraying. Here, we are mainly concerned with clear coats so we will want a needle/tip size of 1.2mm-1.4mm. I use a 1.4mm for shooting my high solids finishes and it is just right. Those using more conventional formulas may find a 1.2mm better suited. My gun of choice is a Devilbiss SRI-pro. It is designed for spot repairs on autos and other small areas and guitars are definitely small areas. Photo 1 shows some of the tools I use in prepping to spray. Stainless measuring spoons (in metric), a small beaker and a mil gauge.
Before we can set up the gun, we must load it with materials. This may just be straight out of the can, as I do for air dry lacquers, or may involve some chemistry like this example where I am spraying an acid catalyzed product. I need about 175cc of material for the project at hand. My mix ratio is 10:1, and since it is really hot and humid today, I will need some retarder/thinner as well. So I measure out 150cc of finish, 15cc of catalyst and add 15cc of a special blend retarder/thinner. This will all be important to know as we proceed. We can now calculate the amount of actual finish resins in our ready to spray mix. We have the standard mix, which is listed at 34% solids by volume, and we have added another 10% thinner, so we now know our RTS mix has 30.6% solids. After mixing well, we can load our gun and start to set it up.
Your gun may have these in slightly different places, but should have all the following adjustments (photo 2):
This will adjust the overall size of the spray pattern. This gun can be dialed down to a ¾” pattern, you may find you need to keep this control almost all the way shut on your gun to get the desired pattern size.
This will adjust how much material can flow through the gun when you trigger it. Smaller patterns & thinner materials will need less fluid. Notice that because there was no Zahn cup on my bench, I am not that concerned with actual material viscosity as I am having the chemistry right for the condition. We will adjust our gun to what we will spray rather than try it the other way around.
This will allow you to dial down the amount of air at the cap to fine tune atomization.
I have a regulator at my gun. This is the best place to adjust air pressure because the hose will cause pressure drop, so settings at the regulator at the other end of the hose are very unreliable.
Now we need a source of good clean dry air. I run an oiless compressor so I have no worries about oil contamination. I have drop traps at many places in my lines to catch water and dirt as well as a filter regulator unit (photo 3). I set my wall regulator to the stated max for my gun. Our first adjustment on the gun will be air. I do this at the regulator on the gun with the air choke wide open. Photo 4 shows that I have just over 80psi at the gun. I pull the trigger to the half position and the gun will flow air but no fluid. I now dial in my regulator to provide constant flow at my desired psi. 28-29 psi is what I want here based on my aircap selection; consult your gun manufacturer’s specs for guidance here.
Now we are ready to set the spray pattern. I use a piece of cardboard to set my gun on. Spray technique comes into play at this point. In spraying small objects like guitars, we want to work in closer with a smaller pattern than if we were doing large panels. A good distance from the surface for this kind of work is 4”, so hold the gun this distance from your test surface (photo 5) and pull the trigger for a split second to create a pattern on the surface. I want a 4” pattern. My first is too large at about 5” and too dry (photo 6). I continue adjusting my pattern control little by little and taking test sprays until I get my 4’ pattern (photo 7). I am also adjusting my fluid control to get the amount of fluid I want. I want a nice, evenly wet pattern to form rather quickly, but do not want enough fluid as to begin to sag. The final adjustment if needed would be the air choke. If you are getting a lot of mist kicking back off of the surface, turn in the choke a little. You should be getting a pattern that forms a nice flat surface with very little texture or “orange peel”. If after adjusting the air choke the surface texture becomes too much, dial in the fluid control a little.
Next, I need to establish my coating weight for the day. To do this, I spray out a test section. I overlap my strokes by 50%, or in this case 2” (photo 8). It is important to establish a rhythm of work. Consistency of application rate, gun distance from surface and pattern overlap are all needed to do this job well. After my test passes I grab my mil gauge and check how much material I am applying to the surface (photo 9). The gauge works by pressing it gently against the wet surface, pulling it away and looking at the edge for how many teeth are wet. Your application weight is between the last wet tooth and the first dry one. Mine today is about 3 mils. This is important, as I can now use that solids percentage from earlier to determine how much dry film will be left and therefore how many coats I need to apply. In this case I am applying about .92 dry mils per coat. With this conversion varnish I want about 3 mils after buffing so I will lay down 4 coats which gives me about .6 mil to level sand and buff: perfect.
Now that we have are gun all set up, we are ready to spray. The best advice I was ever given was to become a machine. To get quality results you must be very consistent with your application. You must keep your application rate or “pass speed” the same, or you will have heavy spots and thin spots in your finish weight. The heavy spots will likely lead to sags, runs or solvent pop and the thin spots could buff through. You must keep the gun square to the surface being sprayed (photos 10 & 11). When the air cap of the gun is square to the surface it will put down an even pattern. When it moves out of square, the pattern will become heavier in one section and lighter in another leading to uneven coating weight and poor surface quality of the sprayed film. This is especially important when spraying around curved surfaces like guitar rims (photo 12).
While not as hard as building a guitar, finishing is a skill. As such it takes careful attention and practice to get correct. Starting with quality materials, using quality modern equipment that is properly adjusted and good technique the job is easier and better results will follow. A clean and proper work environment is also necessary to get good results. Don’t expect to get the best finish in the same dusty shop you built your guitar in. There are many nuances to this type of work and this article only touches on the basics. If you find all this a bit overwhelming, there are those of us out there who will do this part of the job for you.
Originally published in Brian Howard's Guitar Building & Repair Blog.