Orange peel (effect)
Gloss paint sprayed on a smooth surface (such as the body of a car) should also dry into a smooth surface. However, various factors can cause it to dry into a bumpy surface resembling the texture of an orange peel. The orange peel phenomenon can then be smoothed out with ultra-fine sandpaper, but it can be prevented altogether by changing the painting technique or the materials used. Orange peel is typically the result of improper painting technique, and is caused by the quick evaporation of thinner, incorrect spray gun setup (e.g., low air pressure or incorrect nozzle), spraying the paint at an angle other than perpendicular, or applying excessive paint.
In some situations, such as interior house paint, the orange peel texture is generally desirable. In this case, a texture paint is generally applied with a spray gun. The texture is then painted over with the appropriate color. When painting walls, orange peel can also develop by using a roller with too little paint or too thick a paint and the surface dries before the texture can level.
The instruments used to measure orange peel simulate visual perception. Like our eyes, the instruments optically scan the wavy light/dark pattern. A laser point light source illuminates the specimen at a 60° angle and a detector measures the reflected light intensity at the equal but opposite angle. The orange peel meter is rolled across the surface and measures point by point the optical profile of the surface across a defined distance. The instruments analyze the structures according to their size. In order to simulate the human eye's resolution at various distances, the measurement signal is divided into several ranges using mathematical filter functions:
- Wa 0.1 ..... 0.3 mm wavelength
- Wb 0.3 ........ 1 mm wavelength
- Wc 1............ 3 mm wavelength
- Wd 3 ......... 10 mm wavelength
- We 10 ....... 30 mm wavelength
- SW 0.3 ..... 1.2 mm wavelength
- LW 1.2....... 12 mm wavelength
Structures smaller than 0.1 mm also influence visual perception, therefore the instruments use a CCD camera to measure the diffused light caused by these fine structures. This parameter is referred to as "dullness".
The values of dullness and Wa to We form a "structure spectrum". This allows a detailed analysis of orange peel and its influencing factors, being material or application parameters. Example of a "structure spectrum"
- Sherwin-Williams Automotive Finishes - includes comparison photos of orange peel and normal paint