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Thread: Which variable?

  1. #1
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    Which variable?

    I'm still vey much at the beginning of my spray finishing learning curve, pretty well confounded by having too many variables that interact with each other in complex ways. Changing one variable at a time does not seem to be a productive approach, as the variables are not independent. I don't know how to quantify the results well enough to take a fractional factorial design of experiments statistical approach. So asking the experts seems like a reasonable alternative.

    At first blush it seems straightforward; the controllable variables are, I think:

    1) viscosity of finish
    2) pressure of finish delivery
    3) volume of finish delivery
    4) air pressure
    5) air volume
    6) tip size
    7) speed of movement of the gun over the surface
    8) distance of the gun from the surface
    9) fan width

    I'm spraying with a Qualspray AM-6008 HVLP gun. The finishes in question are from the Target line, EM1000, 2000, 6000, 8000.

    My most consistent problem is a dappled appearance, as seen in the picture below. This is taken wet, it flattens out a lot as it dries, but not completely. I bought a thickness gauge to try to see where I was with coating thickness, it worked OK with pigmented finish, I can't figure out how to read it with a clear finish. I can't tell the difference between the probes that are wet and those that aren't. I'm starting with the gun settings per Jeff Jewitt's recommendations.

    I thought that increasing air pressure and/or decreasing finish flow should help, I've managed to create a whole lot more overspray, but it still seems to spit droplets in with the mist, giving this dappled look. I've sprayed gallons of water onto the side of my shed playing with the various parameters; I've not yet found the combination that produces a uniform mist. It's gotten better, but I'm not convinced that this is the best it can be.

    What's the most productive thing for me to try?

    IMG_2809.jpg

  2. #2
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    You'll normally get a little bit of that when finish is wet, but getting more atomization should improve things. To be up-front, I struggle with this a little too, and the fact that I don't spray very often, being a one person, part time, one-project-at-a-time woodworker, doesn't help. Which N/N are you using?
    --

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
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    N/N??

    I'm sure doing it more would help a lot, but I do go weeks between finishing projects. I'm thinking about getting a small touch-up gun that might be more appropriate for turnings, I'd do that a lot more often. I am already stressing about finishing the guitar I'm working on.

  4. #4
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    Just change one thing at a time, and practice on something that's not intended for use. The first thing I would try is, while you have finish in the gun, is to hold the gun closer. If that doesn't do it, and air adjustments don't fix it, I'd try thinning it more.

    I have never been able to lay anything down like glass with anything less than a high end gun, so I can't claim to be very good at spraying. It seems like most people, from what you hear on forums, get as good results with lower end equipment, so I can't claim to be qualified to give advice.

  5. #5
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    You should be able to spray a very nice finish with that gun; it's the same one Jim and I use. The heavy orange peel in your photo shows your coats are far too heavy. The optimum coverage with most WB finishes is just a little less than full coverage when the finish first hits the work. If you overlap your passes by 50% that will fill in 95 - 100%. Then it will flow out after a minute or two to give 100% coverage w/o orange peel.

    I can give you the exact gun settings for my gun with EM-8000CV I just sprayed, straight from the can with no thinning:

    Needle/nozzle set = 1.3 mm

    Air valve to the gun = wide open - always.

    Fan valve = 1.0

    Fluid valve = 1.0 to 1.1

    Air pressure to the cup (PPS H/O in my case but it should not matter) = 5 psi

    Gun inlet air pressure = 29 psi

    Distance of gun from surface = about 8"

    Pace = Hard to say, maybe 1.5 - 2 feet/sec; it's more related to what I said above. Passes overlapping 50% should end up with about 95 - 100% fill.

    You should be able to get a great finish with those settings. The only variable should be pace.

    This is what 4 coats of EM-8000CV looks like with the above settings



    You want a wet mil thickness of about 3 mils. If you have a gage use the edge that brackets that film thickness. Spray a small sample, let it sit for a minute, then press the edge straight down onto the panel and pick it back off. If you can't see the wet finish on the nubs of the gage press it down onto a piece of colored paper and see which nubs leave a wet mark. The largest number that leaves a wet mark is the film thickness. Adjust your pace until you get 3 mils with passes that overlap 50%.

    One more tip. You need as much light on the surface as possible and you absolutely need to be able to see the wet line as you are spraying. Good luck.


    JOhn

  6. #6
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    Thank you John! That is perfect, I will go give it a try. I'd been thinking that I was probably spraying too thick a coat. The hint about reading the thickness gauge is priceless, it's been driving me batty.

    Are you measuring the gun air pressure at the gun or at the compressor side? I've been using a 25 ft 1/4" polyurethane hose with a mini-regulator to step down my house air pressure. There seems to be considerable pressure loss in the hose. I have 3/8" rubber hoses, but they really impede my ability to move the gun smoothly because they are so heavy and unwieldy.

  7. #7
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    I found John's comments helpful, too...I'm going to reduce the pre-gun pressure a little more as I have mine higher and am not always happy with every coat I spray.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  8. #8
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    A good starting point is going to the Coating Manufactures web site and find the Coating that you are using. Look for the products Technical Data sheet, there is a lot of useful information in that sheet from application to viscosity.
    Here is a link for Target Coatings ..Technical data sheets https://www.targetcoatings.com/produ...y-data-sheets/

  9. #9
    I struggle with this too.

    I use a turbine which does not have gauges to set pressure and flow precisely. But I can tell you by feel and some experience that:

    Orange peel is the result of finish not being able to flow out before it dries. This is commonly caused by a too thick coat. But I find that it also happens at colder temperatures for me. Maybe that's because the viscosity increases at colder temperatures. This can also be caused by under-atomization, creating globs that have a farther distance to move to level the surface.

    Spray in warm temperatures. Do whatever you need to do to safely get your finishing space to room temperature. If you are a ninja like Jim or John, you can get by at lower temps by tweaking air and material flow - but it's hard when starting out. For me, in the north east, spraying in my garage between April and October is almost brainless. October-March, it's like Rocket Science...

    You can also add a small amount of thinner to your finish. As long as you use the right material, you will extend your open time this way, which will improve flow at the expense of maybe having to do an additional coat.

    Like sawing by hand, spraying does require a confident hand to spray at the right pace and distance, and eye to recognize film thickness. But temperature is IMHO my biggest nemesis. Get that right first.

  10. #10
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    Quote Originally Posted by roger wiegand View Post
    Thank you John! That is perfect, I will go give it a try. I'd been thinking that I was probably spraying too thick a coat. The hint about reading the thickness gauge is priceless, it's been driving me batty.

    Are you measuring the gun air pressure at the gun or at the compressor side? I've been using a 25 ft 1/4" polyurethane hose with a mini-regulator to step down my house air pressure. There seems to be considerable pressure loss in the hose. I have 3/8" rubber hoses, but they really impede my ability to move the gun smoothly because they are so heavy and unwieldy.

    Get rid of that 1/4" hose. It won't give you the CFM required to drive that gun. That dinky 1/4" hose is likely contributory to the orange peel you're getting. You want at least a 3/8" hose. You'll get used to handling it in no time.

    I have two regulators in my system. One is right at the compressor and is always at 90 psi. The other is at the inlet to my 20 ft hose reel that supplies the gun. That one is set to 29 psi. Before I had the PPS system I had a mini regulator right on the gun, also set at 29 psi most of the time.

    John

  11. #11
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    Sounds like I'm buying a new hose for my gun, too... LOL
    --

    The most expensive tool is the one you buy "cheaply" and often...

  12. #12
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    The Flexilla 3/8" id Air hose is Soft and Flexible. Also use the Full Flow Air Couplers and Fittings for your spray guns.

  13. #13
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    I also like the Flexilla 3/8" hose. The threaded ends freaked me out a little, to start with, since they're plastic, but I haven't had the first problem with them.

    I don't like the almost clear polyurethane hoses that you can see the reinforcement mesh in, for several reasons.

    Milton "H" quick disconnects are on everything I have, short of the big things for 3/4" hose. I'm not saying that you need them, but just by chance was what I started with, and they've served me well for 46 years.

  14. #14
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    The picture shows a Full Flow fitting on the left verses the standard fitting on the right. If you use the larger fitting on a spray gun you will notice a dramatic difference in the way the gun sprays. Its also Best to use the 3/8" id hose (Flexilla)to go along with the bigger fitting. I think the fittings on the Flexilla are Aluminum that has a Black Anodizing on it.
    Air Fittings 2.jpg

  15. #15
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    OK, Flexilla hose on order!

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