How to Choose a Paint Gun

Time to buy a paint gun? Before parting with your hard earned money, read through this brief article to help in your decision process.

With so many choices available, choosing a paint gun or paint guns to can be really confusing. With so many variables and individual needs, I can’t specifically tell you which gun to buy however I will attempt to filter through the many variables and provide a better understanding of what is available and why.

For many years, paint guns were simple devices that used air flowing through small internal passages inside a metal paint gun body creating a vacuum to suck the paint through a tube, out of a canister attached to the bottom and when combined with a needle sliding through in and out through an orifice or nozzle, the paint mixed with air into small droplets becoming atomized then with air pressure, was forced out of the gun, through the atmosphere and landed on a surface, hopefully your car. A bit simplistic perhaps but basically how it was done. These were generally known as Siphon Guns and can be readily identified by the paint canister or cup, attached to the bottom of the gun body. Another similar version looking very much like a Siphon Gun, instead of vacuum, relied on supplied air to pressurize the canister, forcing the paint up through the needle and nozzle, becoming atomized then propelled through the atmosphere. These were known as Pressure Guns or “Pressure Pots” and most often used for production work or conditions where the gun will be inverted part of the time or on extreme angles. Although they were known for producing a beautiful finish in the hands of a knowledgeable painter, both designs required a fair amount of air pressure, usually 50 to 80 PSI to do their job and because they discharged a great amount of atomized paint into the air, were not very efficient, generating a lot of over spray. In some areas of the US, and Europe, these guns are actually not permitted for sale.  With some searching, they can still be found today, however they are considered to be old and inefficient technology.

Another configuration of paint gun layout is the Gravity Feed. Gravity Feed guns are identified by the paint canister or “cup” being attached to the top of the gun body. The main advantage of this design is that the paint is fed to the nozzle of the gun with gravity and since no air pressure is needed to generate suction, it requires less pressure. Added benefits are a shorter path for the paint to travel and much easier cleaning. Also, most are newer designs which take advantage of the great advances in paint gun technology in the past 10 years.

Note: It is a common misconception that if it is a gravity feed gun, it must be an HVLP. Not necessarily. Some of the really inexpensive guns may be made to look like an HVLP but are in fact based on old high-pressure technology. The best advice here is to look very carefully at the gun, the box, any included literature containing specifications and don’t hesitate to ask questions of the seller before forking over hard earned money.

This leads us into the subject of HVLP guns which is an acronym for High Volume Low Pressure. These are by far the most commonly available units today. Ok, you may be wondering what this High Volume Low Pressure actually means. The “Low Pressure” refers to the air pressure required to operate the gun properly. The majority of HVLP guns today require from 15 to 30 PSI (Pounds per Square Inch) which is the force of the air needed to function adequately. The individual gun will state this in the included specifications. The “High Volume” is the CFM (Cubic Feet per Minute) which is the amount of air needed to get the job done. Some guns require as little as 3 or 4 CFM while others will need 10 to 15 CFM. At this point it is VERY important to point out that the PSI and CFM requirements are specified AT THE GUN INLET (I will touch on this in greater detail a little further on in this article). In addition to your main regulator at the air compressor, you need to always have a true gun regulator attached right at the gun inlet to properly measure inlet pressure, they also serve to prevent sudden pressure spikes when opening and closing the trigger.

Another term you may occasionally encounter is LVLP which obviously describes a Low Volume Low Pressure gun. Although virtually identical to an HVLP, the combination of both a lower PSI and CFM requirement will earn a gun this designation. They are generally among the most efficient guns out there and waste little paint however most of them are at the high end of the price scale.

The guns described above are available in full-size as well as smaller versions that you would use for laying a single-stage color or base-clear in a tighter area or over small parts. Named “Mini Guns” by most manufacturers; their usefulness in shooting door jambs or covering small repaired areas is why they are frequently called “Jamb Guns” or “Spot Guns”.

All paint guns share the same basic internal component design that is; internal passages designed for optimal flow, a trigger controlled, sliding “needle” which is a metal rod with a sharp point that extends into and out of the “nozzle” which has a corresponding size to match the needle (they are paired in sets and always used that way) and the “air cap” which surrounds the nozzle and injects air into the paint stream and shapes the spray pattern or “fan”. In addition, they also have assorted springs, ferrules, bushings and seals. The aforementioned “needle” and “nozzles” are sized by the hole in the nozzle where the paint is discharged through. They are measured in millimeters and can be from 1.0 to 2.0 mm with most in the 1.2 mm to 1.8 range. Again, no direct rules apply here as different manufacturer’s nozzle/needle sizes will work better with some materials than others, some basics are; 1.2 mm/1.3 mm for base colors, 1.3/1.4mm for clears and single stage urethanes and 1.5 to 1.8 for heavier-bodied primers and sealers. 1.0mm is what is most often found in a mini-gun.

Earlier in this article I mentioned “air requirements at the gun inlet”. This is a critical point in that all compressors have a CFM rating which will appear as this example; 16 CFM@90 PSI. I stress this as being very important since this rating is at the OUTPUT of the compressor tank, when the compressor is new, with ideal atmospheric conditions, moon in the right phase…you get the picture. Add in ½ dozen air fittings, tank regulator & filter, 50 feet of air hose a few more fittings and if you are lucky, you have 10 or 12 CFM at the gun and if your gun requires more CFM, the paint job will suffer and the gun looks like the bad guy. Note: horsepower is not a good rating of a compressor, always refer to the CFM rating. If you suspect that your compressor may not be up to the task, there are things you can do to help. 1st, use as short of a hose as possible and go with as large of a diameter as you can. A ½” I.D. hose is much better than a 3/8” hose. Try to eliminate fittings if you can, elbows especially. Go with the largest tank Filter/Regulator unit that you can afford. If you need help, there are several excellent online sites that help you calculate air pressure and CFM loss through pipe and fittings. Another important point is that moisture and oil are responsible for ruining probably 70% of paint jobs. You MUST have clean dry air to the gun. Be sure your compressor tank is drained of water, use a good quality moisture and oil trap (in-line desiccant systems work well too). Buy a new air hose and keep it dedicated just for painting to minimize the chance of intruding oil or contaminants to you paint gun.

The controls on most paint guns are quite similar with the air-flow, paint-flow and fan knobs. The fan knob regulates the width of the “fan” or spray pattern which is normally a vertical elliptical shape however this is determined by the horizontal or vertical orientation of the Air Cap. This is determined by the user. Since all guns are different in their base settings, consult the instructions supplied with the gun as a starting point. Each gun design has an individual “feel”. Get familiar with it, waste some paint and practice, practice, practice before tackling that paint job on you old car or truck

There is one other type of painting system that certainly bears mention here and that is the Turbine systems. These are great for those without a compressor or requiring portability. Basically, if you can imagine using the outlet hose of a powerful vacuum cleaner on a specially designed paint gun and heat that air, you have the idea of a turbine paint system. Of course they are a bit more complex and specific than that and cost can vary widely. They provide a huge amount of CFM, the intake air is filtered and the output is heated to provide a dry air supply. I have tested several of them and the better quality units do work quite well. The only caution is that many (but not all) are designed for painting backyard storage sheds and garage doors but not cars & trucks so choose carefully and take note of the included needle/nozzle set size. Some are supplied with 2.0 mm and larger needle/nozzle sets however the higher quality machines offer optional automotive painting sizes.

One final word on cleaning. Failure to take the time and effort to properly clean them has killed more innocent paint guns than any other cause. The cheap, clean-up grade of lacquer thinner or acetone available by the gallon from one of the large discount stores works well. Another alternative is one of the aerosol gun cleaning agents available from many paint and supply houses. To avoid damage, begin cleaning as soon as you are finished painting and minimize the exposure time to the solvent. NEVER leave a paint gun to soak in solvent as it will ruin any plastic internal parts and rubber seals. Drain unused paint from the cup, wipe out as much as you can. Pour a few ounces of the solvent into the cup and spray it out into a rag or wad of paper towels (be careful not to have it spray back at your face and I trust you would still be wearing protective gear at this time). Wipe of any remaining paint form the exterior then remove the air cap and nozzle, put several more ounces into the cup then with the air disconnected, squeeze the trigger and allow the solvent to stream out until it is clear. Put the air cap and nozzle back on Hook the air back up then once more, spray the remaining solvent into the rag.

In summary, shop carefully and hopefully use the information I have provided. You must make the determination on how often you will use the gun, whether to but just one and acquire the additional needle/nozzle sets or buy two or even three separate guns. Many painters have a dedicated color gun, a clear gun and a primer gun. Generally, with the more expensive guns, a great deal of engineering and development goes into optimizing internal flow and atomization characteristics also, they tend to be much longer lasting, these are usually bought by the everyday painter. For the dedicated hobbyist, there are quite a few decent guns and sets available for $200/$300 or so. 

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