Polyspray

[Tomfox]

I am having difficulty with spraying polyspray.

I am using a HVLP gun provided by a person who used to do auto body work. Spray temperature varies from 72F to 82F. This morning was 72F and I chilled the polyspray prior to use. Diluting 4:1 per the spec. I get "chaff" when I spray - bits of coagulated paint randomly distrusted (not all the time). The bits are about 1/32 inch and some as long as 1/4". I am careful to put the spray on wet.
I had no difficulty with the polybrush. The gun appears to be clean. when I spray a test pattern, it seems ok. I am running about 20 psi in operation.


Any advice would be helpful.
Thanks
tom

[jtpitkin06]

HVLP PAINTING TIPS

Filter all of your mixed and thinned paint through a cone strainer. Failure to filter paint is asking for troubles with a clogged tip.

Thin paint to consistency of skim milk. Cream is too heavy and too thick. If in doubt as to what is too thick, use a viscosity cup. Thinner on warm days than cold days.

Before connecting air...

Adjust pattern valve fully open.

Adjust fluid needle fully open. Pull and hold pressure on the trigger while turning the fluid screw out. You'll feel the trigger move back as you turn the screw. When the trigger no longer moves it is fully open.

Connect air to the gun.

Adjust air pressure to value shown on gun. Generally it will be about 25 to 35 psi. Adjust pressure with WITH TRIGGER PULLED so gun expels air but not dispensing paint.


If you have a regulator attached to the gun, adjust gun air valve fully open as regulator will control air pressure. Some guns do not have this valve.

Gun is now adjusted to spray too much paint.

Spray a test pattern on masking paper for two seconds with gun held still about 12 inches from paper.
Pattern should be oval football shaped. If dog bone shaped, reduce fluid. If paint runs, reduce fluid. If paint is a round pattern increase pattern air control or increase air pressure.

The goal is a spray pattern that is oval football shaped with paint that flows and levels at the center. Paint should be feathering at the edges of the pattern.

If paint comes out granulated or stringy, paint is too thick.
If paint cones out spattering pressure is too low.
If paint consistently runs, paint is too thin, operator is spraying too long, or nozzle tip is too big. Generally a 1.3 to 1.4 tip works well with poly spray or poly tone.

Practice on masking paper or craft paper before spraying your airplane. You can throw paper in the dumpster if you screw it up. Fixing a botched paint job on the plane is a lot of work.

If HVLP is new to you, try practicing with water in the gun to see how the controls (pressure, pattern, fluid) work and their relationship to the others. Changing one control changes the other two slightly. Next move on to some cheap practice paint. Spray some lawn furniture or other junk to get the feel of the gun. When you feel confident, move on to the airplane.

Good luck.

John Pitkin
Greenville Texas

[HighWing]

Tom,
You said a borrowed gun. It is a turbine driven pump or conventional air compressor. I used to use one of the low cost turbine driven guns and the compression of the air at the large feed line would heat the air so the actual gun temps were significantly higher than ambient air temps.

I just finished the coatings on an elevator mod I did and got up very early to take advantage of the early morning temps. I also shot the coatings outside as the hangar temps retained some of the previous days highs.

Not absolutely sure, but the chaff sounds like residue from past jobs that are in an incompletely cleaned gun that can soften then loosen then get blown out. I had used the gun I used for fiberglass mold release agent and did find some strings of the dried mold release floating around in the gun when cleaning after the first Polyspray coat. The mold release is waterborne and softened but remained intact. If the gun was previously used for a two part coating, it could soften any residue allowing it to be blown out. It would come out as strings or particles as it wouldn't completely dissolve in the Polyspray.

I have loaned out spray guns and the typical cleaning is to run some solvent through the gun until the spray comes out clear. I find that if I then disassemble the gun, I will get all sorts of residue in the threads of the cup and the various parts. I get better cleaning if I completely disassemble the gun and soak the parts in solvent - usually acetone. I then brush each piece then rinse each piece under fresh acetone and then reassemble the gun.

[Kahle76]

Excellent explanation John! I will use your info next month. Thanks.

[Tomfox]

Thanks for the help. I used the same gun for the polybrush w no problems. I will go through John's process before continuing