When Spraying, Size Does Matter

Dec 3, 2009

With football linemen, basketball players and pro stock drag racing engines, conventional wisdom implies that bigger is inherently better.

For mobile techs, however, when it comes to paint spraying equipment, that’s not always the case.

To see how this is so, let’s look at the three major components that make up the paint spraying apparatus:

  1. The source of forced air. And although this could include turbines, for the sake of this article we will only consider compressors.
  2. The air’s conduit, which is the air channel that begins at the source and ends at the spray gun. This conduit includes all hose lengths, connections, couplings, gauges, dryers, traps, manifolds, L and T fittings. For our purposes, it will also include the overall length and volume of space included within the conduit itself-anything that contributes to airflow resistance and restriction.
  3. The spray gun with its incorporated needle/nozzle setup. And since it appears that many, if not most of us are using high-volume, low-pressure [hvlp] guns, we’ll focus on these.

All three of these components have to harmoniously come together to achieve optimal performance for the painter. [Air quality and condition are also important, and should be considered apart from this article. Our focus here will be on air supply and delivery.]

Let’s look at each component’s role and see how, indeed, size does matter.

The Air You Need

First is the airflow source. For many of us, that means a reciprocating piston compressor, lubricated or non-lubricated, powered by a gas engine or electric motor.

It is important to be aware that the compressor must supply all of the necessary air volume and more than the necessary pressure recommended by the gun manufacturer for your tool to perform optimally.

Since most modern guns don’t require much pressure, supplying it is not usually a problem. That being said, however, never forget that for every inch of conduit utilized, you lose some of your initial pressure. An absurdly long air hose or one that has right angles or too many connections may cause problems.

Now, for air volume. This cannot be ignored, being that most modern guns require much more volume than their predecessors.

A mistake many painters make when choosing a compressor is that they don’t understand that there is a very important difference between cubic feet per minute [cfm] and sustained cubic feet per minute [scfm].

If the volume of air initially provided by your compressor is adequate but not sustained, you will notice that on bigger jobs your spray fan will begin diminishing. As an example, let’s say that a compressor guarantees 10 cfm at 35 psi. Your gun requires 9 cfm at 35 psi. No problem, right?

Well, not if you only spray for 15-30 seconds at a time. After that, however, you will wish you had read the fine print stating that the sustained volume is only 4 cfm, for example-not enough air to finish the job satisfactorily.

The solution for many plugged-in mobile techs is to get as large a compressor as possible that will run on a 110-volt current [and require the fewest possible amps so as not to trip any breakers!] that charges a tank of 15-25 gallons of air. This way you should have sufficient air to not only run your spray gun, but also any other tools that require a lot of air volume such as pneumatic sanders and grinders.

It can be a challenge to find an electric, 110-volt compressor that can provide sufficient psi/scfm if you are using a full-size or older hvlp gun. With newer spray equipment and smaller touch-up guns, however, this shouldn’t be a problem.

As an aside, I only use 10- and 12-gauge extension cords for my compressor. Smaller-gauge cords are usually OK for most other electric tools, such as polishers and drills.

In the case of air source, bigger is usually better.

Room in the Hallway

Next up is your conduit. In a nutshell, what you need to be aware of is that the internal diameter of your hose must be large enough to allow passage of sufficient air volume.

In my experience, this means that a 1/4-inch hose is not large enough for many touch-up guns. You are better off with a 3/8-inch hose.

There are usually numerous connections along the route of your conduit. If these are too small, they can cause a drop in pressure.

One way to ensure they are large enough is to use hvlp plugs and couplers. Another consideration is the pipe that may run between particle, oil and moisture traps. These are often 3/8-inch, so don’t use reducers to connect them.

A couple of illustrations may help you understand and appreciate how volume and pressure differ.

Imagine passing air volume as being a string of medical patients measuring four feet wide being transported in wheelchairs by an orderly through a hospital to the exit. The orderly represents air pressure.

As long as the corridor [your conduit] is straight and wider than four feet, level or better yet gently sloping downward, there is little problem getting where they are going. If the doorways, of which there are many, are only 3-foot-10-inches wide, the orderly can still get the patients through, but he will have to apply additional pressure.

He will also be slowed by sharp turns, and require more applied pressure simply to move his load forward the farther he goes. By now, he’s getting tired and needs another orderly to help. Provided the hallway is still four feet wide, all remains well.

But what if the hallway narrows to only two feet in width? These patients aren’t leaving the hospital without losing a lot of size.

At this point, you might think that just using more pressure will get our patients out of the hospital. But it doesn’t work that way. It’s like being the guy trapped in a falling elevator. He thinks, “Why not just jump up right before the elevator hits the bottom?” It sounds good, but it doesn’t work.

The real downside to compressors producing too much pressure is exploding tanks and air lines. A more likely problem is that you create a moisture issue.

The more air is compressed, the hotter it gets. The hotter it gets, the more moisture it holds. The more moisture in your air, the harder it is to remove. And the more moisture in your paint, the more dye-back and other moisture-related problems with which you have to contend.

A simpler illustration is that of a freight train. The engine is the pressure, while the cars are the volume. The train cannot deliver more in volume than the capacity of the cars. Adding more engines won’t make a difference in volume.

To maximize the engine’s output, railroad tracks usually follow straight, level lines. Grades are gradual and there are no sharp curves, only broad ones.

In the case of your air’s conduit, again bigger is usually better.

Optimum Spraying

Finally, there’s the spray gun. In this case, for many of us, smaller is better.

Smaller guns are easier to clean, require less air, create less overspray and waste, and often cost less than a full-size gun.

But, just as tool choice is important, so is the needle/nozzle setup you select, especially for spraying clearcoat.

As you may already know, the basecoat material is not of the same chemical makeup as the clearcoat. It sprays differently. To me, the only easily noticeable difference between using a 1.0 and a 1.6 needle/nozzle is the rate of dispensed fluid volume.

The advantage of the smaller 1.0 is that it creates less overspray. That means that you can usually get away with using less masking material. Also, I believe a 1.0 to 1.2 needle/nozzle applies the clear much better than a larger setup.

The volume of fluid passing through a smaller orifice is more likely to be correctly atomized by the supplied psi/cfm than when using something larger. The finish should appear cleaner with less chance of runs or objectionable texture when using an ideal gun setup. [This has been my observation using hvlp guns. Someone using a conventional gun or one of the latest-generation compliant models may beg to differ.]

It seems that almost any operation at some point, through increasingly sophisticated advancements, is elevated to the level of being an art and science. In this case, the mobile tech is the aspiring artist. The makers of spray equipment and paint are the scientists.

I’ve tried to interpret the scientists’ work through my own observations and inquiries. And the following seems apparent:

To get the best results when spraying automotive coating with an hvlp gun requires large amounts of air volume driven by moderate to low amounts of air pressure through medium and small needle/nozzle orifices.