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Need some help with spray painting

Old 02-23-2008, 10:46 PM
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JKEpps
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Default Need some help with spray painting

Hi all,

I have an HVLP spray gun I purchased from Harbor Freight. I've used it to paint a plane in the past (over a year ago) and the results came out OK. I had some runs, orange peel, etc... I'm trying to get better at it, but since I spray so infrequently, I'm relearning all the time. I'm building a Balsa USA Ercoupe [link]http://www.rcuniverse.com/forum/m_4687714/anchors_7108129/mpage_3/anchor/tm.htm#7104529[/link], and am covering it with Stits Lite covering. When trying to spray the Poly Brush, per instructions, I got runs, lots of them. I'm pretty sure it's how I have the gun adjusted. What I don't know is the best way to adjust air pressure, spray pattern, material density etc ... for the best results. Can anyone offer some advice/tips or point me to a guide or something.

Thanks,

-Jorden
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Old 02-24-2008, 07:15 AM
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Default RE: Need some help with spray painting

O have not used a hvlp gun but have used a few of the external mix guns and airbrushes over the past 40 years.in most cases the air pressure is set to the working pressure of the gun and the flow is adjusted to that pressure to give a nice mist and pattern on a test piece.a lot has to do with experience or practice and reconizing what causes the problems you are having.Badger used to put out a booklet about airbrushing that showed some examples and had you make adjustments to replicate them so you knew how the gun works and performs and what adjustments to make.orange peel usually results from paont not thinned enough,runs can be several things,paint to thin,volume to high ,not moving the gun fast enough or too close to the work surface.I have painted cars,pick ups and many planes and still am learning.with the change on paints over the years I will be using a hvlp gun on my next spray job
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Old 02-24-2008, 08:33 AM
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Default RE: Need some help with spray painting

Hi Jordan,
I am a full scale mechanic. Having covered and finished three planes using the Stits process, I am a pretty good authority. I also use several HVLP guns.[8D]

What I don't know is the best way to adjust air pressure, spray pattern, material density etc ... for the best results.
With my gravity fed Sharp HVLP, which is very similar to your gun, I run the pressure at 40-45psi. I know that sounds wrong. But, I am telling you, it is the only way to get a decent finish with an air HVLP. (I also have a turbine HVLP). Always keep you gun perpendicular to your work surface. Set you r gun up on a sheet of cardboard, so that yo don't cause runs and sags in your project. Adjust your material flow as low as you can and slowly bring it up to where you can smoothly apply the material without getting it too wet. Too wet, and you have runs and sags, too dry, and the polybrush looks flat as it is going on.
Good Luck!
Doug

P.S. I am finishing a Challenger II Ultralight in the Stits process right now. I have it through a white base coat of Aerothane and will be applying blue and yellow trim this week
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Old 02-27-2008, 07:03 AM
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Default RE: Need some help with spray painting

Doug,

Thanks for the tips. I am glad you mentioned the higher pressure with the gun. In the video, Mr. Mull (I think that's his name) mentions he's spraying at 5 psi. Well I tried that and all I got was splatter. I adjusted evry conceivable option on the gun trying to get it to work at 5 psi to no avail. My gun "looks" almost identical to the one he's using. Actually, the gun shown on Stits website does appear to be identical (for 3x the price from HF right now I might add) to the one I have. I've been shooting at 15 to 20 psi, but my mind kept telling me that was too high. This is an HVLP gun, LP meaning Low Pressure, so I shouldn't need to go higher than 20 psi. Especially since the video shows 5 psi. I think another issue was I used my workshop compressor. It's a small one I basically use to blow the dust off my building area or project. I think it only has like a 3-5 gallon tank, so I empty it pretty quickly. As a result, it runs almost constantly. I try to go slow to give it a chance to catch up, but since the Stits stuff dries relatively quickly, I'm leary of waiting too long. I'll have to bring my larger compressor in from teh garage, but man it's so loud! It's too cold to shoot in the garage, and I can't just run a hose through a cracked door to my work room because of the smell of the Stits products, my wife would skin me alive. I'll give the 40-45 psi pressure a shot. I even thought of using my Campbell Hausfeld touch up gun (non-HVLP) gun as an alternative, but didn't want to deal with the overspray issue in my work room. I'm on pause for the moment anyway waiting for a shipment from F&M. I used up almost a whole quart of Poly Brush on one wing panel as I've had to remove almost all of what I sprayed to get a good even coat/finish. The first (actually) second attempt failed, so I'm having to take it all off again. I think I'm going to have to reshrink some as I've probably loosened some of the Poly Tak. Oh well, it's all a process! even with my trials and tribulations, I'm still liking this stuff. I hope I can make the end results worth the effort.

Thanks again,

-Jorden
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Old 02-27-2008, 02:17 PM
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Default RE: Need some help with spray painting

Jorden:

I sprayed my T-28 with Poly-Lite. While I can't give you any advice directly on the Poly-Brush part of the process (I used Minwax poly to fill the weave and then sprayed Stits FeatherCoat over that to get the paint to stick), I found that I could spray FeatherCoat and Poly Lite at about 30psi indicated on the regulator on the HVLP handle (I used a 1.7mm tip). Tank pressue was 75 to 115 psi (HarborFreight 10gal compressor). The first time I sprayed FeatherCoat I used about 10psi pressure, and had far too much flow coming out of the gun, resulting in lots of runs and drips. Raising the pressure and reducing the paint volume eventually cured that.

The Poly Lite sprayed very well at 30psi. One thing I learned was to not use cardboard to test the spray pattern and flow rate: it absorbs the paint faster than a treated surface, so what looks good on cardboard may run on your fuselage. The other thing I found was that if you do get a minor run, wait a couple of hours before you try fix it: the paint evens up pretty well as it drys.

You should like the end result.

Fred
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Old 02-27-2008, 02:41 PM
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Default RE: Need some help with spray painting

One thing I learned was to not use cardboard to test the spray pattern and flow rate: it absorbs the paint faster than a treated surface
I have been using cardboard boxes to set my spray patterns for over 20 years. It works great for me. What do you use?
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Old 02-28-2008, 03:42 PM
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Default RE: Need some help with spray painting

FirstPlace:

You've got a lot more experience than I do, so I'll be happy to learn from you. What I found was that when I did test sprays for pattern, coverage, etc. with pieces of cardboard, the paint would go on smoothly with no runs or drips... but when I'd spray it on the wing, fuselage, etc. it would run. I thought it was because of the harder surface presented by the fiberglass.

Anyway, I had some scrap pieces of hardboard (masonite?) left over from another project that had a smooth, hard surface, so I used them for test spraying. I adjusted the paint volume and pressure settings until I got a smooth spray pattern, then I'd paint the plane.

Another factor that's probably involved is that I was spraying in less than optimal temperatures... it was around 30* ambient temperature, so I set up a couple of heat lamps inside a makeshift spray booth that probably got things up to around 45*. The PolyLite sprayed fine, but I'm certain that spraying at higher temperatures would make things go better. A friend of mine that has srayed automotive paint says that one of the advantages of these higher end paints is that you can spray on a wet coat and let if flash off, resulting in a smooth finish. When I was spraying, I ended up with multiple light coats because of the drip/run problem and the cold.

I noticed a couple of other things with the PolyLite: (1) It seemed like the thickness didn't build up the way I thought it would. Trying to sand off any sort of blemish seemed to expose the undercoat very easily. (2) When I masked off the wings to paint the wingtips orange, I used a couple of layers of paper to mask off the areas secured with masking tape. But when I unpealed the tape and lifted the paper, there was a very light amount of orange that seemed to have seeped through the paper onto the white areas. It'd clean up with some paint thinner, but I eventually started masking stuff using plastic transparent film.

I've got two other planes to spray once it warms up, so any further advice you have is much appreciated.
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