Troubleshooting Spray Stripes in Lacquer Finish

      Thoughts on what causes a spray pattern to "stripe," and advice on how to fix it. May 23, 2007

Our finisher just finished a kitchen and bath in a dark brown mahogany on red oak. When he shaded the doors, they had spray stripes which show, and with a trained eye I noticed the stripes. The customer has not said a thing about the stripes, only that the color looks great. Our finisher used a SprayTec 9200 turbine with cup HVLP gun and Valspar 550 voc lacquer. Can the finishing stripes be fixed?

Forum Responses
(Finishing Forum)
From contributor G:
If your customer thinks it looks great, don't disillusion them.

Streaky Lacquer Problem

From contributor C:
If the stripes are in the lacquer, just respray with some retarder in the lacquer and a wet coat will fix it. Sounds like resprayed too light and needs a wetter pass.

From the original questioner:
Should we still use the turbine, or go to a pressure pot setup to spray on a wet coat? I think the hose on the turbine is too hot for the lacquer, which caused the stripes on the finish.

From contributor C:
It's urban legend that turbines warm the lacquer - they don't. I sold Accuspray for years, and sold a couple of other brands.

From contributor A:
I agree with the first comment don't mention anything if the customer likes it. Speaking from experience, it's a bad and costly road to go down. Your problem could be as simple as the spray tip needs to be cleaned after every use of the gun. If your tip is blocked slightly, your fan spray becomes uneven, causing fingers or stripes on your finish. Been there before. It's a simple fix.

From contributor R:
"It's urban legend that turbines warm the lacquer - they don't. I sold Accuspray for years, and sold a couple of other brands."

Are you saying that turbines don't heat the air? I think that breaks some of the laws of physics. My air compressor sure does get warm when it is running. I sure am glad the compressor is 150 feet away so the air cools off enough to get the moisture out. I've seen turbine guns that have a cushion on the handle because they get so warm. It has been many years since I've used one; maybe the technology has changed. I'm pretty sure the air is warmer coming out the turbine when the hose is only 20' long, compared to my gun that has its air supply 150' away.

From contributor J:
"My air compressor sure does get warm when it is running. I sure am glad the compressor is 150 feet away so the air cools off enough to get the moisture out."

Actually, air gains moisture as it cools. Even if a turbine does provide warmer air, it won't do much to warm the finish at the point of atomization.

From the original questioner:
The customer has noticed the stripes and brought it to our attention. I think we will try the suggestion of respraying with some retarder in the lacquer.

From contributor T:
Hmmm... a bit of hot air here. The laws of thermodynamics tell us that air gets hot when it gets compressed (it doesn't care how it gets compressed) and it cools when it expands. So if a turbine compresses air and it gets hot, what temperature is it when it expands at the air cap? And why is it so dry in the cold far north while it is so humid in the hot deep south? But I digress.

I'm with you on wanting to fix it. Several things can cause stripes.
1. Be sure your finish viscosity is consistent with your spray setup. If it's too thick, marginal atomization can entrap micro-bubbles of air in the finish.
2. Use a retarder. If the finish skins over prematurely, you can get micro-bubbles of evaporated solvents trapped in the finish.
3. Be sure you get a consistent wet mil thickness. If your finish sheen is anything but gloss, the flatting agent will settle to the bottom of thick areas and remain near the top of thin areas. That causes differences in sheen which can appear as stripes.
If it's micro-bubbles you've got, they're hard to get rid of. If it's just inconsistent finish thickness, a consistent wet coat should fix it. Thinner is actually better.

From contributor J:
Personally, I never cared much for turbines. Give me a pressure pot or an air assisted airless any day.

There are a number of things which can cause the striping effect:
Too thick a viscosity for the spray setup.
Incorrect solvent used for temperature conditions. (Too fast evaporating.)
Too much air pressure at the gun.
Rushing the spray job, not keeping the passes close enough together.
Inexperienced sprayers. I see this all the time when cabinetmakers try to spray. They are so afraid of getting runs, they don't spray wet enough.

What caused the striping in your case? It could be any one or more of these things. On the bright side, you and your finisher will learn from the experience.

From contributor R:
Distance of the gun from the work and width of spray pattern will also cause this pattern. Place a light away from the work and use it to reflect off the fresh coat while applying. This will let you see the wet coat, or lack of it, better than an overhead light.

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