Causes of Orange Peel | Popular Woodworking Magazine
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Understand and avoid this common spraying flaw.
ORpeel of the range is the most universal flaw in a sprayed finish. It is an irregularity on the surface that resembles the skin of an orange – hence the name.
Regardless of the brand or quality of spray gun you’re using, or how well you clean and tune the gun, or what you’re spraying, you can still get orange peel. You want to avoid it if possible.
The problem is that there are so many causes (and therefore so many solutions) that eliminating the problem can become very confusing.
Here is the list of causes from a manufacturer:
■ Liquid material is too thick
■ Failure to deposit a wet hand
■ The spray gun is moving too fast
■ Insufficient air volume
■ Insufficient air pressure
■ Wrong fluid nozzle
■ Incorrect air cap
■ Hold the spray gun too far away
■ Hold the spray gun too close
■ The thinner evaporates too quickly
■ Too much air flow
■ Too high temperature
■ Too low temperature
■ Humidity too low.
There are many causes. Everything can be correct, but there are so many of them that I bet you stopped reading after the first two or three and skipped to this paragraph. The trick is to squeeze these causes into just three or four so you can keep them in mind. This can be done as follows:
■ The liquid is too thick for the amount of air
■ The gun is moved too fast across the surface or held too far from it
■ The liquid dries too quickly
■ The gun is held too close to the surface.
These four can be reduced to three by eliminating the last one. Holding the gun so close that it causes the finish to ripple is so obvious and rare that a separate instruction is not required to indicate not to. We take each of the three remaining causes in turn.
Viscosity and air
The most common cause of orange peel is an imbalance between the viscosity of the finish and the amount of air that atomises the liquid coming out of the gun. The denser the liquid, the more air is required; the thinner it is, the less air is needed.
So the obvious solution is to increase the air pressure or dilute the material.
Increasing the air pressure is the most efficient way because there is no solvent waste or finish loss. Both compressors and turbines produce enough air to atomize most finishes under normal conditions. But the turbines are limited on the upper side if you need more air than normal. Compressors are not limited. It is possible to increase the air pressure far beyond that obtained from a turbine.
However, thinning is often the simplest method of reducing orange peel. But the price is small, which may make you need to apply more coats.
There are two further possible solutions: heat the finish before spraying or use a smaller diameter fluid nozzle and corresponding needle.
Liquids are more viscous, that is, denser, at colder temperatures and less viscous at warmer temperatures. Keep this in mind because when it’s cold, your finish will likely be thicker in the morning unless you keep the thermostat on all night.
There are several methods of heating a finish. These include putting the finish container in a larger container of hot water or using a bucket warmer or even an electric blanket (if you’re careful) wrapped around the finish container. You can also use an oven or microwave if the amount of finish is small.
Smaller diameter needle / nozzle sets spray less fluid material, so less air is needed to achieve good atomization. However, there are two problems. These sets are generally quite expensive and may not even be available for your spray gun. And most importantly, they will cause your spray pattern to shrink and slow down your production.
Speed and distance
The second most common cause of orange peel is moving the gun too fast across the surface or holding it too far from the surface. The result is that the surface does not get wet completely, so the finish cannot level out. There is no proper speed or distance because there are too many variables: how quickly the finish dries, what volume of finish comes out of the spray gun, whether the pattern is narrow or wide, etc.
The goal is to spray a coat that’s wet enough so that the finish comes out and levels well, but not so wet from puddles or sagging. The only way to know the correct speed and distance is to observe what is happening in a reflected light.
This is more difficult when spraying complex vertical surfaces. It is also more difficult to organize sufficient lighting. But working with reflected lights is probably the most important rule in finishing, whether sprayed or brushed, and it’s rarely mentioned.
For best results, organize your location and lighting so you can see how the finish performs at all times. Reflection is always necessary to see the development of the orange peel.
A finish needs time to level before starting preparation. If the equilibrium between the temperature and the evaporation rate of the solvent (or solvents) is not correct, there may not be enough time for leveling. You will need to lower the temperature in the spray area or add a slower evaporating solvent.
Lowering the temperature, usually with air conditioning, can be quite expensive because cooler air is exhausted so quickly. Diluting with a slower solvent, or retarder, is usually the easiest way. But this, of course, may make you need to apply multiple coats.
One step after the orange peel caused by the finish drying too fast is the “dry spray”, which has a sandy texture. The solution for dry spray is to dilute the finish with retarder or keep the gun closer to work.
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