Furniture Finishing & Restoration
Spray Finishing Problems
"Orange Peel" is a defect in the finish caused by improper spray application technique. The surface of the finish has the look and feel of an orange. There are a number of causes for orange peel; though a very slight, almost imperceptible orange peel is normal; look closely at a car's factory paint job and you'll see it.
Here's a list of common sources and the fix;
Needle/Nozzle wrong size
Select and use the correct needle/nozzle size based on the coating you are spraying. The spray gun manufacturer/supplier and/or the coating manufacturer/supplier can recommend the best size. Generally, a smaller needle/nozzle atomizes the finish better but reduces fluid flow and fan width. The thicker the finish, the larger the needle needed.
Insufficient air pressure/volume
Adjust the atomization air pressure to the minimum setting needed to produce a fine spray. Too little atomization air pressure will produce larger droplets and a center-weighted spray pattern. Too much air will produce excessive overspray, dry spray, and/or the finish will dry before it flows level. Make sure the compressor supplies enough air, in cubic feet per minute (CFM) for the spray gun you're using.
Viscosity of coating
Thin the finish, as needed, to achieve the proper viscosity. Properly thinned, the finish should atomize into tiny droplets that flow out and level. If the finish is too thick, it will spray in larger droplets. If it's too thin, you will get sags/runs unless you spray very thin coats.
Adjust the fluid flow to get the best atomization at the current air settings. Too much fluid flow for the amount of atomizing air will result in large droplets (poorly atomized). Too little fluid will dry before flowing level.
Spray gun distance/speed
Maintain the recommended distance from the spray surface. Depending on the spray gun and settings, a constant distance in the range 6" - 10" inches is generally best. Maintain a steady hand speed during each spray pass, from end to end, that lays down an even wet coat in the thickness range recommended by the coating manufacturer. Use a wet mil gauge to measure the wet film thickness.
In hot weather, the finish can dry before flowing out. Use a slower evaporating thinner (retarder). Excessive air flow over the sprayed surface will cause the finish to dry before flowing out. Avoid strong air current over the drying surface.
There's some information on measuring viscosity and wet film thickness at this link - Measuring Viscosity & Wet Mils.
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