Obtaining a perfectly smooth surface of the body, reminiscent of a mirror, is the highest aerobatics in body repair, which not every master can achieve the first time. Many beginners are faced with the problem of the so-called “orange peel” or shagreen, which spoils the appearance of a freshly painted part and requires labor-intensive sanding. Understanding the physical processes involved in spraying paints is the key to success and avoids costly rework.
The main cause of defects lies in the incorrect balance between the rate of solvent evaporation, material viscosity and spray parameters. If the paint is applied to the surface already dry or, conversely, too liquid, it does not have time to spread into an even layer. In this article we will look at the technical nuances that will help you paint without shagreen and get professional results even in garage conditions.
The quality of the final coating directly depends on the preparation of the base and compliance with the temperature conditions in the spray booth. Ignoring even one parameter, such as air humidity or pressure at the outlet of the gun, can negate all efforts. Let's take a closer look at each stage of the technological process to eliminate errors.
Physics of the process and causes of defects
Shagreen is a surface microrelief that is formed due to the fact that drops of paint do not have time to merge into a single monolithic layer before polymerization. The viscosity of the material plays a decisive role here: if it is too high, the surface tension prevents the drop from spreading, and it solidifies in the form of a hemisphere. That's why viscosity control is the first step towards an ideal paint job.
The second critical factor is the rate of solvent evaporation. When using a thinner that is too “fast” in a cold room, the top layer of paint sets instantly, trapping the more liquid fractions inside. This creates a ripple effect, which is visually perceived as roughness. On the contrary, a slow solvent in hot weather can cause smudges, but often helps the material spread better.
⚠️ Warning: Never try to fix shagreen by adding solvent directly to the spray gun tank while working. This will disrupt the proportions of the components and may result in discoloration or loss of adhesion.
The third aspect is the quality of air preparation. The presence of moisture or oil in the air flow inevitably leads to the formation of craters and irregularities that are disguised as shagreen. The use of high-quality filter-moisture separators and regular purging of the main line is a prerequisite for working with any acrylic enamels or varnishes.
It is also worth considering the distance to the surface. If you hold the gun too far away, some of the solvent evaporates into the air, and already overdried paint gets onto the part, which physically cannot spread. The optimal distance is 15–20 cm, and deviation from it by more than 5 cm is already considered a critical violation of technology.
Why is shagreen visible only after drying?
Many people notice the defect too late. The fact is that the liquid film of paint has a high refractive index and hides irregularities. Only after the solvent evaporates and the material shrinks does the microrelief become visible to the eye. That is why it is important to control the process immediately after applying the first layer.
Selecting and setting up painting equipment
The tool you use determines 70% of the success of the operation. Cheap spray guns with poorly polished nozzles and an unstable spray practically give no chance of obtaining OEM-level results. To work with modern materials, it is recommended to use guns of the system HVLP (High Volume Low Pressure) or LVMP, which provide high material transfer and soft atomization.
The nozzle size must strictly correspond to the type of material being applied. For base metallic enamels, a diameter of 1.3–1.4 mm is considered optimal, while for thick acrylic varnishes and primers a nozzle of 1.6–1.8 mm is required. Trying to force thick varnish through a fine nozzle will lead to increased pressure and the formation of fog, which will intensify the shagreen.
The pressure adjustment should be made directly at the moment the trigger is pressed (in an open torch), and not statically. The standard value for most materials is the range of 2.0–2.5 atmospheres at the head exit. Exceeding this value crushes the material into too small particles, which dry out during flight.
Adjusting the shape of the torch also affects the uniformity of application. For horizontal surfaces, the torch is opened as wide as possible to ensure soft spraying. For vertical surfaces or complex profiles, the torch can be slightly clamped to control the flow and avoid over-dusting.
Preparation of paint and varnish material
Even the most expensive spray gun will not save the situation if the material is prepared incorrectly. The key step is to select a solvent that matches the temperature in the spray booth. Manufacturers produce thinners labeled "Fast", "Standard" and "Slow", and the choice should depend on environmental conditions.
Viscosity is a parameter that must be measured and not determined by eye. Using a viscometer (funnel) allows you to accurately determine the flow time, which is usually 16-18 seconds for bases and 20-22 seconds for varnishes at a temperature of 20°C. If the viscosity is above normal, shagreen is practically guaranteed.
- 🧪 Mix the components (base, hardener, solvent) thoroughly for at least 2-3 minutes until smooth.
- 🌡️ Bring the temperature of the material to room temperature (20°C) before mixing, as cold paint has a different viscosity.
- ⏳ Maintain the “viability” of the mixture: prepare as much paint as you can produce in 40–60 minutes.
- 💧 Use only solvents recommended by the manufacturer, do not experiment with cheap analogues.
Filtration of the finished mixture through special mesh filters (“watering cans”) is mandatory. Even a microscopic speck that gets into the nozzle can disrupt the geometry of the torch, which will lead to streaks and uneven spraying. This is a simple operation that is often overlooked, but it is critical to quality.
If you are working in a cold room, slightly warm up the paint itself (can) by placing it in a bucket of warm water (no higher than 30°C). Warm material spreads better and produces less shagreen.
Layering technique and process control
Proper painting technique involves applying the material perpendicular to the surface at a constant speed of hand movement. Jerks, stops or changes in the angle of the gun lead to local changes in the thickness of the layer, which causes different degrees of spreading and, as a result, non-uniform shagreen.
The first coat (wet or dry, depending on the type of paint) is applied thinly to create an adhesive base. Subsequent layers should be more saturated. It is important to comply with the intercoat exposure specified in the technical data sheet of the product. If you apply the next coat to an under-dried base, the solvent may “boil” and create bubbles.
Particular attention should be paid to blocking passages. The standard is to overlap the torch by 50% for each new movement. This ensures even distribution of the material and hides the transition boundaries. Insufficient overlap creates streaks (“bull-eye”), while excess overlap leads to overflow and leaks.
⚠️ Attention: When painting large areas (hood, roof), start moving your hand before pressing the trigger and release it after completing the pass. This will eliminate thickening at the beginning and end of the line.
To control the thickness of the wet layer, you can use a special comb tool or simply visually evaluate the gloss. The material should lie flat, without tears or excessive fluidity. Experienced craftsmen feel this moment through the pistol handle and visual control of the torch.
☑️ Control of layer application
Temperature and conditions in the chamber
Temperature and humidity in the painting area are parameters that are often underestimated and considered secondary. However, they are the ones who dictate the rate of chemical reactions. The optimal temperature for painting is considered to be 20–22°C. At lower temperatures, the material becomes viscous and flows poorly.
Air humidity also plays a role: at humidity above 70%, there is a high probability of clouding of the varnish (the “whitening” effect) and the appearance of matte spots. In such conditions, it is necessary to use special anti-silicone additives or evaporation retarders to give the material time for proper polymerization.
Drafts and working supply and exhaust ventilation during paint application can create chaotic air currents, carrying part of the torch to the side and drying the material on the fly. During painting, ventilation is often switched to recirculation or low-pressure mode to ensure a calm atmosphere.
The stability of the room temperature is more important than its absolute value. Sudden changes in heat or cold during drying are guaranteed to lead to surface defects.
Parameter selection table for different materials
To systematize data on setting up equipment and materials, it is convenient to use a pivot table. It will help you quickly navigate depending on the type of chemistry used and environmental conditions.
| Material | Nozzle diameter (mm) | Pressure (Bar) | Solvent type |
|---|---|---|---|
| Base enamel (Metallic) | 1.3 – 1.4 | 2.0 – 2.2 | Fast / Standard |
| Acrylic enamel (Solid) | 1.4 – 1.5 | 2.2 – 2.5 | Standard |
| Acrylic varnish (2K) | 1.6 – 1.8 | 1.8 – 2.0 | Standard / Slow |
| Soil filler | 1.8 – 2.0 | 2.5 – 3.0 | Standard |
Using inappropriate parameters indicated in the table can lead not only to shagreen, but also to other defects, such as “coarse” shagreen (at low pressure) or “dust” (at high pressure). Always check the technical data sheet for the specific product, as compositions may vary from manufacturer to manufacturer.
Removing shagreen: polishing or repainting?
If it was not possible to avoid a defect, the question arises of eliminating it. Fine shagreen can be removed by polishing with abrasive pastes after complete polymerization of the varnish (usually after 24 hours or after heating in the chamber). This process requires the use of water-cooled sanders and a gradual progression from P1500 to P3000 abrasives and finishing pastes.
However, if the shagreen is deep ("orange peel" to the touch), polishing may remove too much varnish, leading to rubbing and loss of gloss in the future. In such cases, the only correct solution is repainting the element. Removing the old layer, thoroughly sanding the base and applying a new layer of varnish in compliance with all technologies is a guarantee of durability.
If you do not eliminate the factor that caused the shagreen (for example, the wrong solvent), the next time you paint, history will repeat itself. Analyzing errors after each painted element helps improve your skills.
Is it possible to add solvent to the varnish to remove shagreen?
Adding a solvent to a varnish that is ready for application is possible, but in strictly limited quantities (usually no more than 5-10% of the volume). Exceeding this norm will disrupt the chemical structure of the film, the varnish will become soft, lose chemical resistance and may not dry completely. It is better to choose the right thinner from the beginning.
Does shagreen depend on the color of the paint?
Yes, indirectly. Black and dark colors (black, dark blue) visually emphasize any, even the smallest shagreen, making the defect obvious. Light colors (white, silver) better hide surface unevenness due to the characteristics of light reflection. Therefore, the requirements for the quality of application on black cars are always higher.
Does the brand of spray gun affect the results?
Undoubtedly. Professional pistols (Sata, Iwata, DeVilbiss) have precision-machined needles and nozzles, as well as stable aerodynamics of the air channels. Cheap Chinese analogues often have backlashes and an unstable torch, which makes obtaining an ideal result without shagreen a very difficult task even for an experienced painter.
Do I need to sand the base before varnishing?
The acrylic base (“metallic”) cannot be sanded; the varnish is applied directly to it after the solvent has evaporated (haze). Sanding the base is only possible if it is a special insulating primer or if it is necessary to remove defects in the base itself, but then re-application of the base is required. Usually only acrylic enamels such as “Solid” or primers are sanded.