Setting up and spraying with an HVLP gun

Setting up and spraying with the Harbor

Freight HVLP gun #38308 and the 43430


Pressure Adjustment: Open your compressor regulator up so there is plenty of air pressure in the line to the gun (I set mine to about 90 psi). Next set the regulator on the gun handle to 50 psi (what I use for DP40 and K36) with the trigger pulled and leave it set there. Close the valve at the bottom of the handle (on the gun, not the regulator) and then with the trigger pulled, open it to the point where the air volume starts to stay the same (just listen to it) and leave it there (it should be pretty far open at this point). I never mess with this after this unless I think it has been changed.

Note: On my two year old gun they say the maximum pressure is 60 psi. On my new gun it says 40 psi. Instead of the 50 psi above you might try 40 psi if your gun says 40 psi max.

Fan Adjustment: Next open the fluid volume control about 3 turns for starters (at the back of the gun). Close the fan control down (on the left side of the gun behind the nozzle) so when you spray you only get small round circle. With the gun about 6 to 8 inches from a test surface (I use the garage door or better yet some sheet rock, or masking paper) move the gun across the surface (about 6 to 8 inches from the surface) and pull the trigger. As you are moving turn the fan control until you get the desired fan shape/width (about 5-6 inches wide top to bottom oval shape for overall painting) (remember if you are spraying up and down on a surface you can change the nozzle at the front of the gun 90 deg. so the fan is horizontal).

Fluid Control: Next after getting the fan the right size continue spraying and adjust the fluid volume at the back of the gun for the final adjustment to get the fluid right for the gun speed (how fast you sweep it across the work). You want to be laying on the paint at a pretty good volume, but not causing runs at a normal gun sweep speed.

Changing the pressure to match spec sheets: Now you’re ready to paint and with 50 psi at the handle the gun is spraying about 10 psi at the cap. If you want 5 psi at the cap put 25 at the handle regulator (8 psi = 40 psi, 7 psi= 35 psi, etc.). You can’t actually see the pressure at the cap which is what the spec sheets are referring to. The gun is internally regulating the pressure down based on the inlet pressure. Some expensive guns have a gauge off the back of the handle that tells you the cap pressure and then they also have a regulator and gauge at the bottom of the handle like this gun that regulates the pressure into the gun.

Painting: Look at what your painting and the way the paint is going on the surface right behind the gun and adjust from there. For instance if you want to spray some small areas/parts you can turn the fan down to a small round size. You will also have to greatly shut the fluid volume down at the back of the gun (clockwise) at the same time to avoid runs. I usually turn the air pressure at the regulator on the handle down to around 25 to 35 psi to do spot work like this.

Proper Pressure: Remember you need a high air pressure (Set as the spec sheet calls for) so that you get proper atomization and so you can turn the volume up. The higher pressure will actually mean fewer runs since the atomization is better. This is not so critical with the primers, but you really need it with the sealer, base, and clear coats so you get good coverage without runs and with little orange peel.

How should I properly set up my HVLP spray gun?

How should I properly set up my HVLP spray gun? [by: MARTINSR]
Hotrodders Bulletin Board: Knowledge Base: Body-Exterior: Articles

Being HVLP (High Volume Low Pressure) and low VOC (Volatile Organic Compound) products are the way the industry’s going, I will be referring to them in this discussion on painting and paint guns. Most all basic issues dealing with HVLP can be applied to conventional guns, atomization is atomization. The HVLP just arrives at it differently.

The object of the spray gun is to break up the primer/sealer/paint/clear (I will call this “PSPC” from here out) into small particles and lay them in neat little rows on the panel being PSPR’ed. So, the whole outcome rests on how well the gun is doing this. Picture the droplets of PSPC coming out of the fluid tip of the gun and then the air “slapping” them into smaller droplets.

You have two things that help you with this process: air and solvent. Solvent can mean something that is already in the PSPR from the manufacturer or something the manufacturer has told you to add to it. By the way, you should always mix in proper ratios as instructed in the tech sheet. The thinner (less viscosity) you get the PSPR or the more air you have at the fluid tip of the gun the more it will break up the PSPR. The target for you is getting the perfect balance needed. Too much solvent and the PSPR will have no body, fill, durability, etc. Too much air and you blow the PSPR everywhere but the car, and you’ll have poor adhesion, excessive texture, etc.

So, the answer is proper air supply and gun (and fluid tip) choice, and how you adjust it.

With today’s high-solids low-VOC (Volatile Organic Compound, you know the bad stuff that goes up into the air we breathe) products, there is less solvent. And with HVLP guns there is less air at the cap to break up the PSPC. Proper air supply and gun setup is more important than ever.

FIRST THINGS FIRST — your compressor and air supply.

An HVLP gun requires more VOLUME of air to operate (the V in HVLP, High Volume Low Pressure). Now you may notice that your HVLP gun is adjusted at maybe the same PSI as an old conventional gun, around 50 lbs at the gun (many HVLP guns are set at much lower though) so where is the “Low” in PSI they are talking about? It is at the actual air cap where the air and paint come out. An HVLP gun has only 10 lbs at the cap while a conventional has upwards of 50! So the VOLUME of air (CFM — Cubic Feet per Minute) is the key to proper atomization with an HVLP.

If you have a gun that requires 15 CFM you will need a compressor and plumbing that will produce that at a very minimum. There are HVLP guns that need as little as 7.5 CFM so you can get good results even from a smaller compressor. Air supply is a complete subject by itself, so let’s assume that you have the air supply needed and move on to gun setup.

So atomization is the key, but why? Why can’t you just lay it out wet and let it “flow”, as an old painter will say. Picture a jar full of bb’s; they will represent small, atomized droplets of PSPC. The gaps in between the bb’s is solvent. Now picture a jar filled with marbles, they will represent large, poorly atomized droplets of PSPC. The gaps in between are, you guessed it, solvent.

If you apply your PSPC in large poorly atomized droplets, what you will have is a film full of solvent. This can and will cause slow curing, shrinkage and dieback (the loss of gloss in the hours and days after application).

So, now that we have learned the need for gun setup, how do we do it? Let’s start with the fluid tip choice. The newer high-solids low-VOC PSPC products need to be broken up more, so a smaller fluid tip is needed.

Basically you want the smallest fluid tip that will still allow you to PSPC the particular part you are PSPC’ing, keeping the entire thing wet and in a fair amount of time. In other words, a 1.0 tip would be beautiful for clearing one fender, but would be lousy to paint a complete. The application would be way too slow, and the first panel would be way too flashed by the time you got back around to it. So you need to compromise — a 1.3 is a great all-around tip, while a 1.5, though getting a little big, can get you by. If you read the tech sheet on the particular product you are shooting, it will have a recommendation for fluid tip size.

There are needs for other tips. For instance, when shooting polyester primer you may need as big as a 2.3, but for urethanes and epoxies, the 1.3 or 1.4 will work great. If you plan on using a pressure pot or painting a bus, all bets are off and we would need to study a little bit more.

As an example of the use of a 1.3 tip, I did a test once that proved the point well. I shot two panels of metal with a medium solids urethane primer. One was shot with a 1.3 super high atomizing top of the line topcoat gun. The other was shot with a 1.5 (or a 1.7 I can’t remember) “hoser” primer gun. Three coats were applied and after a full cure (the one shot with the larger gun took MUCH longer to flash and cure by the way) the film thickness was measured. The one shot with the 1.3 tip was 2 tenths of a mil thicker! The larger gun laid out the marble sized droplets full of solvent, and when the solvent flashed the film shrank.

You need to “tune” your gun EVERY TIME you use it, just as you would tune a guitar before you perform. This is done with a very basic sprayout pattern test. This very basic test tells you how your gun is atomizing and you adjust it to achieve the best atomization you can.

Let’s do a spray pattern test:

Set the fan width as needed (you don’t want to change it after you have “tuned” the gun). Turn out the material knob about 2 1�2 turns. This is the “mixture” adjustment, kind of like the idle screw on a carburetor. The farther in it is screwed the lower the fluid to air ratio is and the smaller the droplets will be. The farther out it is, the higher the fluid to air ratio is and the larger the droplets.

Set the air pressure at the inlet to the gun to the manufacturer’s specs. On an HVLP gun, this spec is usually found on the gun, and is the maximum PSI it can have while still maintaining the maximum 10 lb at the cap for legal HVLP transfer efficiency (68 %). You are now ready to do a test sprayout.

Tape a piece of masking paper on the wall for the test. Hold the gun at a right angle to the wall, just as if you were going to paint the wall. Hold the gun at a spread out hands distance (about 8 inches, or 22 centimeters). Pull the trigger to completely open for a split second and then close it. You want an ON-OFF wide open-completely closed in ONE movement. You should have a cigar shaped pattern with complete coverage in the center with fading coverage going away from the full coverage cigar shape in the center. The center should be fully covered without any runs. If you have runs, either you are holding the trigger too long, you are too close, or the gun is simply applying too much material; in which case you need to screw in the material knob or turn the air pressure down. But most likely if you have turned the material knob out the 2 1�2 turns and the air is set at the factory specs, you are just too close or holding the trigger open too long.

The droplets you see trailing off the center are what you will use to “tune” your gun.

Turn in the material knob to make the droplets smaller (and or raise the air pressure). The balance you need to attain is the smallest droplet size possible before you lose the coverage desired. In other words, if you turn in the material knob too far, not enough material will be coming out to cover the panel!

Now, you’ll notice that I said, “raise the pressure to the gun”, while earlier I said to set it to manufacturers specs. We are talking a very small adjustment. It is a fine balance in material-to-air ratio and a little more air than specified is okay. Even if it is an HVLP gun the inlet pressure recommended is to maintain the 10 lb limit at the cap. Well, about three quarters of the country has no regulations for HVLP use, so if you go over the 10 lbs all it will do is atomize the material a little better. You may lose a little of the benefits of HVLP though. But remember you have a lot of control with the material adjustment knob.

After you are happy with the droplet size, DON’T TOUCH THE FAN CONTROL. It will change the PSI at the cap and will change the atomization you worked hard to get.

Do this sprayout every time you spray as material change, temp, and humidity will necessitate a sprayout droplet pattern test. Good luck!

Check out this link to see an example of a sprayout pattern.

Do it yourself questions: how to use a hvlp spray gun

Do it yourself questions: how to use a hvlp spray gun
When using an Hvlp spray gun the painter should be aware of a few things. These are pressure adjustment, fan adjustment, fluid control, changing the pressure to match the spec sheets, painting, and proper pressure.

To adjust the pressure, open the compressor regulator to about 90 pounds per square inch (psi), so there is enough air pressure in the line to the gun. Now, pull the trigger on the gun and set the regulator on the gun’s handle to 50 psi or what is advised in the instructions for the gun. Close the valve at the bottom of the gun handle and while continuing to pull the trigger the trigger open the valve on the bottom of the handle until the air volume remains steady.

Adjusting the fan is accomplished by opening the fluid volume control, located at the back of the gun, about 3 turns. Close the fan control located on the left side of the gun behind the nozzle. Position the gun 6 to 8 from a test surface, move the gun across the surface and pull the trigger. While moving the gun, adjust the fan control until the desired fan shape, usually a 5 to 6 inches wide top to bottom oval shape, is accomplished. If spraying up and down on a surface, change the nozzle at the front of the gun 90 degrees to create a horizontal fan.

After adjusting the fan, continue spraying and adjust the fluid volume located at the back of the gun for the final adjustment. This adjustment is necessary for correct fluid for the gun speed. This needs to be a heavy volume, but not to the point to where the paint runs at normal gun sweep speed.

After completing all these adjustments, the gun is ready to paint. With 50 psi at the handle, the gun is spraying about 10 psi at the cap. If 5 psi is required at the cap, set the handle regulator at 25 psi. Most guns internally regulates the pressure down based on the inlet pressure while some more expensive guns have a gauge located off the back of the handle that reads the cap pressure.

Observe the item painted and the way the paint is going on the surface right behind the gun and adjust from that position. For instance, to spray a small area, turn the fan down to a small round size. The fluid volume will also have to be greatly reduced at the back of the gun at the same time to avoid runs. The air pressure at the regulator on the handle can be reduced to 25 to 35 psi to accomplish this task.

Remember that a high air pressure will be needed, as per spec sheet, so that the proper atomization is reached and the volume is raised. The higher pressure will lower the amount of runs because the atomization is better. This is important with sealers, bases and clear coats so that a good coverage without runs can be accomplished.