how to set air pressure, control thickness for spray painting

[Paulh Tremblay]

I am completely new to spray painting. I just finished my first experiment spray painting with an HVLP gun and a compressor. I am spraying a General Finish high gloss Poly, tinted black. My test cabinets are MDF.

I had several problems, but I think I should focus on two basic principles, controlling my air pressure, and controlling the thickness of my finish.

How does one set the air pressure for a spray gun? I did it this way: I set my air compressor at 60 PSI. I closed my control flow valve and fan valve, and pressed the air gun trigger. The needle on the control gauge at the base of the gun dropped. I adjusted the knob until the air pressure was about 29 PSI, the recommended pressure. Without the trigger on the gun pressed, the air regulator shows a much higher pressure than 29 PSI.

Getting the thickness of the finish correct really proved difficult. The thickness is supposed to be between 2 and 5 Millimeters. I sprayed a thick piece of paper on a horizontal surface as a test and used my wet mil gauge. It seemed I couldn't get a thin enough finish. Every time I used the wet mil gauge, it showed a thickness well above 5 mm.

Finally, I got a thinner coat by moving the gun pretty fast across my work. This technique produced a finish seemingly to thin. None of my experiments produced a nice glassy surface, but I also didn't prep my wood correctly. I used a shellac with wax as my sealer, and yes, the first coat just slid off. (Luckily these are just practice carcasses. Lesson learned.) I sanded the carcass and applied two more coats, but my finish seemed more flat than glossy, and it seemed to show the sanding beneath.

I am not sure on the size of my nozzle. I ordered my gun from Jeff Jewit (and I have his book as well). The gun comes with an additional nozzle of 1.5 mm and 2.5 mm. I am not sure of the nozzle on my gun.

[Lonnie Gallaher]

PT,

A couple of observations. First I am not an expert a using an HVLP gun and compressor, although at one time I was pretty good with a conventional cup gun. So I am still trying to figure out the HVLP gun and compressor. What I would do is call Jeff and he can help you set up your gun. In the mean time this is what I would try. I do not know what gun you have but 60 psi line pressure sounds high to me. My gun states 40 max, if I remember correctly. My experience is that the gun pressure is set without the trigger pulled. So I think you are using too high pressure at the gun.

This is just for your information, the paint thickness gauge is calibrated in mils. This is thousands of an inch. It is not a metric calibration. 5mm would be 3/16 of an inch thick.

So to address the film thickness, I would turn the trigger adjustment all the way in so that the gun is spraying the least amount of material. I would turn the adjustment screw out until the film thickness is to your specifications. During this setup process you will also need to adjust the fan to provide the proper coverage.

You did not mention what gun you have. That will help others to respond.

I am finishing cabinets at this time with General Finishes High Performance. It is a Poly. My gun is a Wagoner HVLP cup gun. I am using the #2 nozzle, I don't remember the size of that nozzle - 1.3mm comes to mind, but I am not sure. 40psi line pressure at my water trap and about 16psi at the gun regulator. This is seat of the pants, but the material adjustment was about 3/4 turn out from fully closed and the fan was about 8 inches at about 8 inches from the surface.

I hope this helps. But calling the pro - Jeff Jewit will net you much better advice. Of course practice is the only remedy for poor performance along with the proper equipment and instruction.

[Paulh Tremblay]

Thanks for the response. My gun is a qual spray gravity gun. I contacted Jeff, and he told me the nozzle is stamped on the side of the nozzle, and it was: 1.8 mm. I used a Ford viscosity cup and determined I would need a 1.3 mm nozzle, which my gun did not come with.

I used the 1.5 mm nozzle for some more testing and got better results, but the paint still went on too thick and sagged. It seems like I will have to hold the gun pretty far back. But I will also need to get either a different gun or a different nozzle set if I want to use GF finishes.

[David Hawxhurst]

there is a happy medium for distance i use 6"-8" form the surface. your right about it sagging if its to thick. to lay on a thinner coat move the gun faster or reduce the amount of material coming out of the gun. if the gun is to far from the surface you'll end up spraying a dry coat, material is drying before it can lay down on the surface and adhere and flow out properly.

[John TenEyck]

Which GF product are you using? If it's Enduro Black Poly, the viscosity is 60 - 70 seconds through a #4 Ford cup. You would probably need an orifice of least 2 mm for a gravity feed HVLP gun. I regularly spray GF's Enduro Clear Poly (40 - 50 seconds) with a 1.8 mm orifice HVLP gun/compressor and have no trouble. But it would not be suitable for the higher viscosity Black Poly without thinning. Unless you have a pressure assisted gun, a 1.3 mm orifice would be way too small for the Black Poly.

I set my compressor at 90 psi and then regulate the pressure down with the mini regulator at the gun inlet. For the Enduro Clear Poly I set it around 28 psi. I set the fan wide open, and the fluid flow around 2 - 2.5 turns open. I spray about 8" off the work, and adjust the speed of the gun to get a wet coat. Practice on wood, plywood, MDF, etc., not paper. Unless your paper is coated it will have a far different absorption and what you get with it won't translate when you switch to wood. The only thing I use paper for is to adjust the fluid flow and air pressure of the gun to get a good pattern. I also don't use a mil gage; I let the work tell me what to do. Sags or runs means I'm not moving fast enough. Orange peel means I applied too heavy a coat, even if it didn't sag/run, or the gun pressure was too low. A rough surface means I was moving too fast, held the gun too far away from the work, or there was a lot of over spray falling back on the work.

Practice, observe, adjust. Repeat.

John

[Paulh Tremblay]

Which GF product are you using? If it's Enduro Black Poly, the viscosity is 60 - 70 seconds through a #4 Ford cup. You would probably need an orifice of least 2 mm for a gravity feed HVLP gun. I regularly spray GF's Enduro Clear Poly (40 - 50 seconds) with a 1.8 mm orifice HVLP gun/compressor and have no trouble. But it would not be suitable for the higher viscosity Black Poly without thinning. Unless you have a pressure assisted gun, a 1.3 mm orifice would be way too small for the Black Poly.

I set my compressor at 90 psi and then regulate the pressure down with the mini regulator at the gun inlet. For the Enduro Clear Poly I set it around 28 psi. I set the fan wide open, and the fluid flow around 2 - 2.5 turns open. I spray about 8" off the work, and adjust the speed of the gun to get a wet coat. Practice on wood, plywood, MDF, etc., not paper. Unless your paper is coated it will have a far different absorption and what you get with it won't translate when you switch to wood. The only thing I use paper for is to adjust the fluid flow and air pressure of the gun to get a good pattern. I also don't use a mil gage; I let the work tell me what to do. Sags or runs means I'm not moving fast enough. Orange peel means I applied too heavy a coat, even if it didn't sag/run, or the gun pressure was too low. A rough surface means I was moving too fast, held the gun too far away from the work, or there was a lot of over spray falling back on the work.

Practice, observe, adjust. Repeat.

John

Thanks. I will try your advice.

I just looked at my can, and I actually have pre-cat urethane. (I thought I was using poly before I checked.) According to the tech sheet:

https://generalfinishes.com/sites/de...ane-060310.pdf

The viscosity is 30-45 seconds.

My viscosity was 18 seconds. I assume I am using a Ford #4 cup. That is what I got from Jew Jewitt's store. I did add one ounce of trans tint dye, so I don't know if that lessened the viscosity, though I can't imagine it would alter it that much. I have emailed Jeff, and he now believes I have a bad can of product, so he gave me the email of a rep to replace my can.

The link above recommends a 1.1 mm nozzle for both the poly and precat.

As far as the air pressure goes, this video:

https://www.youtube.com/watch?v=HDuuJvyiZiI

recommends you set the air pressure at the wall (in our case, at the air compressor) closer to what you really need. I'm not sure if that is good advice or not.

[John TenEyck]

I generally measure the viscosity to be within the range stated by GF, or higher, never lower. Run some water or some other reference liquid through your viscosity cup and see how it compares to the standard. I've never seen a bad can of GF product, but if it got frozen it would definitely be bad, and probably look it, too.

A viscosity of 30 - 45 seconds would be ideal for a 1.5 - 1.8 mm orifice HVLP gun. GF's recommendation of 1.1 - 1.3 mm is misleading. I know they state it's for an HVLP gun, but what they fail to mention is that they used an air assisted setup. I have no idea why they fail to mention that. Anyway, your 1.8 mm orifice should be perfect.

I know others set their compressor at some lower pressure than I do. I leave mine at 90 psi because I run other things off of it besides my spray outfit and it's convenient to do so. I've never had any issues with secondary regulation at the gun, so I see no reason to lower the setting at the compressor. If I did have poor secondary regulation I would lower the regulator pressure at the compressor.

John

[Rick Mosher]

When spraying are you opening the fluid all the way? (pulling the trigger all the way open) A lot of beginners turn the fluid control almost all of the way in so you can barely pull the trigger and assuring that you don't get too much fluid and try to avoid runs and sags. I prefer to leave the control all the way open and learn to "feather" the trigger to get the proper spray results. This allows me to move faster or slower or open and close my fan as required for different spray operations without worrying about the fluid settings. I think you should be able to spray your coating at that viscosity with the tip you have. Try closing the fluid adjustment almost all the way closed and see if it helps. Then try and teach yourself to pull the trigger that much with the control open all the way. Also what is you fan adjustment? Is it as wide as possible or very narrow? A wide fan is the best for spraying clears.