Finishing Techniques: The Basic Concepts Of Adjusting An HVLP Spray Gun And Some Tips On Cleaning A Spray Gun

By Bruce W. Maki, Editor

Start:

Spray finishing with an HVLP spray gun can be highly productive and give superior results, or it can be frustrating and quickly ruin a painting or wood finishing project.

The key, I believe, is understanding all the variables involved (some of which are adjustments to the spray gun) and having enough patience to make small adjustments and observe the results.

The Many Variables Of Spray Finishing:

Air Pressure Regulator: The red arrow points to the air pressure regulator adjustment knob. When the knob is turned in (clockwise), the pressure regulator will maintain a higher pressure, which can be seen on the pressure gauge. I usually set the pressure regulator between 18 and 22 PSI. A higher pressure setting results in better "atomization" of the liquid, which just means that the spray mist is finer because the droplets are smaller. However, higher pressure also results in more overspray.

Note that the air pressure entering this spray gun cannot exceed 50 PSI, so I adjust the pressure regulator on the air compressor down to 50 PSI. Excessive pressure at the inlet side of the gun can damage the gun's pressure regulator. Other spray guns may have a different maximum pressure.

The Main Spray Gun Controls: 1: Air Volume Adjustment 2: Material Volume Adjustment 3: Fan Width Adjustment

1) Air Volume Adjustment:

When this knob is turned fully clockwise, the air flow is shut off. It takes some experimenting to find the right air flow setting. Too much air flow can make excessive overspray and cause a rippling effect in the liquid that has already been applied to the surface. I understand that excessive air flow can also cause the wet surface to dry too fast, which can create problems in the final results like blush.

Too little air flow can result in improper atomization, which may be noticed as spots of liquid on the surface. You may notice that the gun is "spitting" liquid instead of making a nice cloud of spray.

2) Material Volume Adjustment:

This knob is on the top rear of the spray gun. Turning the knob clockwise closes the valve and reduces the amount of liquid that flows from the cup into the gun. If the material volume is too low, the spray stream will be weak and it may take a long time to apply the finish. If the material volume is too high, the problems of inadequate air volume may show up because you are basically trying to atomize too much liquid for the volume of air flowing through the gun.

Note that the air volume and material volume can be adjusted together. By that I mean there may many settings of air volume and material volume that will give excellent results. When both valves are opened slightly, you basically have a low capacity spray gun, which could give results similar to an "airbrush". An airbrush is essentially a small-volume sprayer for fine detail work. In theory you can use an HVLP sprayer in place of an airbrush, but the tool is much heavier so it's much more difficult to control and get the precision results that are possible with an airbrush.

When both air and material volume are increased, the spray gun can reach it's maximum capacity in how much surface can be covered per unit of time. Open up these valves a lot, and you can spray a lot more square feet per minute. But either air volume or material volume may still need to be tweaked to give the best results.

3) Fan Width Adjustment:

This control, which is usually on the side of the gun body, adjusts how much the spray stream is "flattened" into an oval shape. With the fan control fully off (turned clockwise) the spray stream will create a round spot. A wide oval spray pattern lets you cover more area quicker, but if the fan is too wide the droplets of liquid may not "melt" together properly, resulting in a lumpy surface commonly called "orange peel".

If the fan width is set too small, it will take more side-to-side passes to cover the area being sprayed, but the bigger problem is the fact that it's easy too apply too much liquid and have problems with runs and sags.

Note that the fan spray pattern can usually be adjusted from vertical to horizontal, or any angle in between, simply by loosening the air cap (the removable cover on the front of the spray gun) and rotating the nozzle. I usually leave the spray pattern at vertical, but a horizontal pattern is useful if I'm spraying something tall and narrow.

Tip: When I'm adjusting my spray gun, I try to get a cloud of mist similar to what comes out of a quality can of spray paint or urethane.

Other Variables:

There are other "adjustments" involved in spray finishing that don't involve the spray gun.

Thinning Of The Liquid:

When the liquid is thinned (diluted) with a solvent it's easier to atomize, but thinning also changes the speed of drying which has a whole set of consequences. Too much thinner can also create problems in the final results, like blush or "solvent pop". I understand that coatings like polyurethane should be thinned (with mineral spirits) no more than 10 per cent.

Distance From Spray Gun To Work Surface:

The advice I've heard is to keep the spray gun tip about 12 inches (30 centimeters) from the surface being sprayed. However, this distance can be altered, and sometimes it's difficult to maintain the desired distance. If the distance is too great, the droplets of liquid can start to dry before they hit the surface, and then droplets won't melt together, leaving a pebbly surface or orange peel.

If the distance is too close, the coating may be too thick, which can cause runs and sags and other problems from drying too slowly. I've read that the air flow from the gun may create problems (like rippling) when the distance is too close, but I haven't experienced these problems.

Speed Of Travel:

How fast you move the spray gun from side to side will effect the amount of liquid applied on a given area. Slow travel means a heavier coat. Fast travel means a lighter coat but possibly problems with droplets blending together. When spraying stain, the results may be a speckled appearance. In theory, too fast of a travel rate could create a lumpy appearance in urethane or paint, because the droplets are too far apart too blend together. I can't say that I've ever had a noticeable problem from moving too fast when I've sprayed urethane or automotive paint. In general, too fast of a travel rate seems to cause fewer problems than moving too slow.

Amount Of Overlap Between Passes:

The general rule is to make each pass overlap the previous by 50 per cent. Easier said than done. I don't use my spray guns very often, so I get out of practice. Sometimes it can be difficult to keep the overlap constant, or to achieve that 50 per cent overlap.

Too much overlap will apply a heavier coat, which might cause runs and sags and other problems.

Too little overlap can create bands or streaks where the material isn't thick enough. This is a problem when spraying paint or stain, it's not a problem when spraying clear finishes like urethane because you can always cover that flaw with the next coat (unless it's the final coat).

Ambient Air Temperature:

Ideally, spray finishing should be done around room temperature. When this is not possible, some professional painters will substitute different solvents when diluting their coating.

In cold weather, xylene (also called Xylol) can be used as a thinner. Xylene (pronounced "zye-leen") is quite volatile and evaporates easily. Xylene is a common ingredient in carburetor cleaner and is highly flammable.

In hot weather, VM&P naphtha can be used as a thinner. Naphtha evaporates slower than mineral spirits, so in hot weather it may help prevent the coating from drying too fast Naphtha is used for lighter fluid in old-fashioned lighters (such as Zippo lighters). If you've ever used such a lighter, you'll may have noticed that naphtha doesn't burn as fast as other solvents or fuels. Both xylene and naphtha can be purchased at paint stores, hardware stores or major home centers.

Personally, I have not tried using these alternative solvents. I would prefer to wait for better weather to do my spray finishing. This is considered an advanced technique and should be approached with caution.

Tips On Cleaning An HVLP Spray Gun:

When I'm done spraying, I will pour the unused liquid back into it's container. Then I pour some solvent into the spray gun cup, swish it around, and connect the gun to the air hose. I then spray against a piece of paper or cardboard for a few seconds until I can see the solvent spraying out.

Then I disconnect the air hose and take the spray gun outdoors for cleaning, if possible. I typically put the spray gun in a plastic dishpan, empty any solvent from the cup, and then dismantle the gun for cleaning.

The first part I remove is the air cap. Normally this can be removed by hand. Wearing chemical-resistant gloves, I will soak this in solvent and clean it with a small stiff-bristle brush (with plastic bristles, not metal).

Then I remove the nozzle with the multi-wrench (included with the spray gun). On both of my spray guns this requires the 19mm wrench opening.

Once loosened, the nozzle can be unscrewed. It helps to pull the trigger while doing this, which retracts that needle. I soak this part in solvent and clean it with a brush.

Then I unscrew the material cup, which usually requires that multi-wrench. I usually clean the cup last.

Under the cup is a little screen filter. I remove the filter and clean it in solvent. Replacement filters should be available at most auto body supply stores.

After I remove the filter, I hold the spray gun over the dishpan and pour some clean solvent down that filter hole.

Then I lay the gun in the solvent and use a small bottle-brush to clean the areas indicated by the arrows: The liquid passageway The threads for the air cap The inside area around the needle. Be careful... if you bend that needle it will need to be replaced, along with the nozzle.

After rinsing everything with clean solvent, I blow all the parts dry with compressed air, or at least wipe everything dry with paper towels.

I pour the used solvent into a container marked "used paint thinner" or whatever solvent it happens to be. Used solvent should be treated as hazardous waste and disposed of properly.

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Written January 15, 2009