Luthier Tips du Jour - Spray Guns

A spray gun can be a wonderful tool for applying a finish to your instrument.
However, it can also be your worst nightmare if you don’t know how to set it up and use it properly.
Some advantages of using spray guns are: faster application of material,
uniform coverage which results in a more even finish with less work to rub it out
and you can do subtle color effects for toning and shading also known as a sunburst.
Also almost any liquid finish can be sprayed.
Some disadvantages are: wasted material –
not all of the product ends up on the surface of your project.
Some of it becomes overspray which has to be removed from the workplace somehow,
and expense – outfitting yourself with spray equipment can be expensive depending on your set up.
There is a wide variety of spray systems available today for the hobbyist
to the professional, but they all work on the same principle.
The liquid product is atomized into tiny drops as it is mixed with pressurized air.
The spray gun determines how the liquid is atomized
and directs the mixture onto the surface
where it flows back together to make a continuous film.
There are two basic types of spray guns:
conventional high pressure guns
and the newer HVLP or high volume low pressure guns.
Conventional guns use high pressure from a compressor
to apply the product to the work piece.
These guns atomize the finish well
which means it flows out and levels into a smooth finish.
However, they are not very efficient as only a small percentage of the material
actually ends up on your project.
HVLP guns were designed to improve transfer efficiency
so that more material actually ends up on your project
instead of around your shop and in the atmosphere.
Just like the name says, air is used at a higher volume with less pressure
so that less finish bounces off your project and into the atmosphere.
HVLP guns can use two different air sources: a turbine or a conventional air compressor.
If it uses a compressor it is known as a conversion gun
because it converts the high air pressure from the compressor
to a lower pressure as it exits the tip of the gun.
There are basically two designs for these spray guns: siphon feed,
where the cup is on the bottom and the materials is fed up into the gun with pressure
and gravity feed where the cup is on top of the gun
and gravity is used to feed the material into the gun.
No matter which type of gun you use it is imperative that you take precautions
to protect yourself from the solvents during spraying.
Make sure to use a respirator with organic cartridges
and replace them whenever the product can be smelled through the mask.
I prefer to use a full face shield when spraying
so that my eyes are also protected from the solvents.
Make sure that the area you are spraying in is well ventilated.
This could be in a spray booth specifically designed for removing fumes
or outside, preferably on a day when the wind
carries the fumes and overspray towards your neighbor’s house instead of yours.
Seriously though, you can’t be too careful
when working with dangerous finishing products.
Spraying a guitar or anything for that matter
requires and certain amount of familiarity with your spray gun
as well as the product you are spraying.
If you are spraying for the first time I recommend spraying water or solvent
while adjusting the controls on your spray gun.
Start by closing all the controls on the back of the gun by turning them clockwise.
Hold the gun and slowly begin opening the fluid control knob on your gun.
A fine round mist will appear and gradually become denser
the more you open the fluid control knob.
Next, if your gun has one, begin opening the air control knob.
The spray pattern will get wider as you open it.
Once you have control of these two aspects
practice spraying on scrap to determine how far
or how close you need to be from the work piece.
Depending on how well the product is atomizing
will also help you determine how slow or fast to move the gun across the surface.
Next, try putting some finish material in the gun and practice with it.
If you don’t see a fine mist coming out the tip
then trying thinning the product by adding solvent.
You can also increase the needle size or increase the air pressure.
Try holding the gun about 8 inches from the surface
and moving the gun slowly from left to right or vice versa.
Make adjustments with the fluid and air control knobs
until you see a uniformly wet swath on the surface.
Make another pass overlapping the previous one by 50 percent.
Keep your gun the same distance from the surface as you go.
Do not move your arm in an arc or radius.
Move it straight across the surface.
You can also try holding the gun really close
as you move across the surface.
You will notice that the air leaving the gun
will leave what is known as an orange peel or texture in the finish.
Next try holding the gun farther away from the surface
and you will notice a dry rough finish.
After set up and testing I begin spraying my guitar.
I usually start with the back making sure to overlap each pass by 50 percent.
I then spray one side of the guitar
making sure to keep the gun the same distance from the side as I go.
I then do the other side.
Next, I spray the top.
Notice that I have placed a sponge with newspaper over it in the soundhole
so that the finish doesn’t get into the guitar.
Now comes the fun part.
I hold the guitar through the soundhole with my right hand
and spray the neck with my left hand.
This can take a bit of practice
and you need to be careful to not touch the wet finish around the soundhole as you hold the guitar.
If I did everything correctly I should have uniform coverage
that is smooth with no runs or drips.
The good news is that you will be applying several coats
so the first few coats are used to dial in your set up and your technique.
Most of the problems encountered while spraying like uneven patterns,
too much overspray, orange peel and dry and rough spray
can be solved by adjusting the fluid and air control knobs,
adjusting the distance from and rate of movement across the surface as you spray
and the viscosity of the product you are using or a combination of all these ingredients.
In addition you need to regularly clean your spray gun.
A clean spray gun prevents a lot of problems encountered while spraying.
If I am applying several coats of a quick drying finish like lacquer or shellac
I don’t clean my gun between coats
and even then I clean it by just spraying some solvent through the gun.
For water and oil based products as well as catalyzed products
I thoroughly clean the gun after each use.
It will take a little practice but the ideal set up and adjustment along with your spray technique
will give you a smooth even coat of finish.
I refer to this process as a Tango.
You are in constant movement, always adjusting the fluid and air control knobs
as well as the distance you are from the surface as you spray
according to the viscosity of the product you are using.
On a good day it really is like a well rehearsed Tango.
Other times you need to play with the adjustments until you get it right.
Unfortunately every product behaves differently
and even humidity and temperature can play a role in this dance.
Now that you know the basics though, with a little practice
you should be able to get professional results.
To learn more about spray guns and how to use them
I recommend researching what Jeff Jewitt has to say about the subject.