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Old 03-06-2005, 11:21 AM
Brian Martin,Freelance adviser
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Originally Posted by Montypierce
I just shot a test panel with Omni MBC and Omni clear. The base went on great but the clear ornage peeled terrible. I am using a craftsman HVLP with 40 psi at the gun (max 43) and 1.5mm tip. The clear requires no reducer and a 2:1 mix with hardener. This clear is quite thick. I have a 1.7mm tip for this gun. Should I have used the larger tip?

Monty, do not use the 1.7 tip, it will create trapped solvents in the clear and probably more orange peel anyway. I would say "try it" being you are doing test panels, but it could make for a smoother surface and could be mistaken for a better job. However, as I said, it will not atomize the clear enough and you will end up with trapped solvents only to cause die back and shrinkage later.

The very first thing I would think about is the air pressure. Believe me, if it says 43 lbs, you need AT LEAST 43 lbs, and a little more is even better. I say this without knowing what compressor and air system you have. I assume you are at home with a smaller compressor and possibly an improper piping system. This usually means inadequate CFM. The "V" in HVLP is "Volume" CFM is volume, the "H" is "High" so you need a High Volume of air for the HVLP to work. If your compressor can't keep up with the volume you are simply not going to atomize your clear. So "bumping up" the pressure (the "P" in HVLP) is likely needed to also bring up the CFM.

“Basics of Basics” Atomization
By Brian Martin

Being HVLP and low VOC 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 PSPRed. 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 manufacture or something the manufacture 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, 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! This 10 lbs at the cap is something you have no way to measure. “AT THE CAP” means at the fluid/air cap where the paint sprays out. It takes a special air cap with a gauge on it. This “test cap” is used by paint reps, air quality control agents but only a few painters. I have never seen on in a body shop. I say this because the instructions that come with many HVLP guns tell you to “adjust to 10 lbs at the cap” all the while they give you no way of measuring the cap pressure. There is usually a “MAX PRESSURE” stamped on the gun or in the instructions. This “max pressure” is the maximum pressure you can set the gun pressure and still keep the cap pressure at the 10 lbs max at the cap the law allows (in most VOC regulated areas where HVLP is required). 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 it’s self so lets assume that you have the air supply needed and move on to gun set up.

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 well 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 set up, how do we do it? Lets 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 PSPCing 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 to slow and the first panel would be way to flashed by the time you got around back 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 many urethanes and epoxies, or base coats the 1.3 or 1.5 will work great. If you plan on using a pressure pot or paint 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 med 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.

This however doesn’t mean I recommend a 1.3 tip for your primer gun. It is only meant to make the subject of atomization easier to understand. Always refer to the tech sheets of the particular product you are shooting to see what tip you should be using.

Air supply is a subject that could fill many pages by it’s self. So we are going to assume you have that covered and move on to gun set up.

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 spray out pattern test. This very basic test tells you how your gun is atomizing and you adjust it to achieve the best atomization you can.

Lets do a spray pattern test:

Set the fan width as need (you don’t want to change it after you have “tuned” the gun). Turn out the material knob about 2 ½ 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 manufactures 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 spray out.

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 the wall. Hold the gun at a spread out hands distance (about 8” or 22cm). 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 ½ 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 loose 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! That balance of atomization and enough material coming out is what you are after.

Now, you’ll notice that I said, “raise the pressure to the gun”, while earlier I said to set it to manufactures specs. This can be anywhere from 25 lbs to 50 lbs. This gun manufactures “recommended pressure” is AT THE GUN. This means at the INLET of the gun, NOT at the cap and NOT at the other end of the hose, but AT THE GUN. If your gun doesn’t have a regulator on it, then install one. This regulator will tell you at all times what the pressure is AT THE GUN, which is what you need to know. Be sure to buy a quality regulator from a paint gun manufacture. Some cheapie regulators are going to rob air volume from your gun because of restrictions within them. This is something most home hobbyist CAN NOT afford do to small compressors. If you don’t have the air volume to atomize your paint, you are NOT going to get satisfactory results.

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

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