Obtaining a perfectly smooth surface of the body, reminiscent of a mirror, is the highest aerobatics in the painting business. Many craftsmen spend years honing their skills to understand how to apply varnish without shagreen on a car, eliminating the appearance of the characteristic “orange peel”. The problem lies not only in the quality of materials, but also in a combination of factors: from air preparation to hand movement technique.
Shagreen is a microrelief formed due to uneven spreading of the material or its premature drying in flight. To avoid this, viscosity, temperature and pressure must be strictly controlled. In this article we will analyze the physics of the process and give practical advice on setting up the equipment.
Physics of the process: why shagreen appears
Understanding how varnish molecules behave when sprayed helps prevent defects. Shagreen is formed when drops of material do not have time to spread into a single film before polymerization begins. Viscosity plays a key role here: material that is too thick lays out in lumps, while material that is too liquid can cause drips, but often spreads better.
The second important factor is solvent evaporation. If solvent disappears too quickly, the surface of the varnish “seizes”, fixing unevenness. This often happens when the temperature in the spray booth is high or when using fast thinner in hot weather.
The third aspect is the quality of torch crushing. The spray gun should break the material into tiny particles of the same size. Large drops create a relief that will not have time to level out. Therefore, the inlet pressure spray gun must be strictly adjusted according to the product data sheet.
Also (not to be ignored) is the condition of the substrate surface. If the base or primer has a large scratch or roughness, the varnish will only repeat this relief, enhancing it with its gloss. Ideal smoothness begins with high-quality sanding of the previous layers.
⚠️ Attention: Never try to remove shagreen by simply adding more solvent to the finished mixture by eye. Violation of the proportions of the hardener will lead to the fact that the varnish will never dry or will become covered with bubbles in a month.
Preparation of equipment and selection of materials
Choosing the right tool is 50% of success. To apply varnish (2K), professionals use HVLP (High Volume Low Pressure) or LVLP (Low Volume Low Pressure) spray guns. They provide a soft application with minimal mist. The diameter of the varnish nozzles is usually from 1.3 to 1.5 mm, which allows even thick compounds to pass through.
Air quality is a critical parameter. The line should be free of moisture and oil. The moisture separator should be located directly in front of the gun handle. Even microscopic drops of oil that get into the varnish will create craters that cannot be removed by polishing.
When choosing a varnish, pay attention to its characteristics. There are materials with varying degrees dry residue (HS, MS, UHS). High concentration (HS) varnishes give a thicker layer in one pass and flow better, but require high skill. For beginners, materials labeled “anti-shagreen” are better suited, although they do not forgive gross mistakes.
Do not forget about the cleanliness of the instrument. Before adding varnish, rinse the spray gun with clean solvent and blow with compressed air. Residues of old material or dust in the channels can impair the stability of the torch.
Mixture preparation technology and viscosity
The most important stage where most mistakes are made is preparing the working mixture. Varnish and hardener must be mixed strictly by weight or volume, using measuring cups. The use of "Coke cans" is unacceptable if you want to get a predictable result without shagreen.
Viscosity is the resistance of a liquid to flow. A viscometer (funnel) is used for measurement. The measurement is carried out at a temperature of +20°C. If the temperature is lower, the viscosity increases and the varnish lays down like shagreen. If it is higher, it becomes too liquid.
The table below shows approximate viscosity values for different types of varnish (according to DIN4):
| Material type | Viscosity (sec DIN4) | Nozzle diameter (mm) | Pressure (bar) |
|---|---|---|---|
| Varnish MS (Medium Solid) | 16-18 | 1.3 - 1.4 | 2.0 - 2.5 |
| Varnish HS (High Solid) | 18-22 | 1.4 - 1.5 | 2.0 - 2.2 |
| Varnish UHS (Ultra High Solid) | 22-26 | 1.5 - 1.6 | 2.2 - 2.6 |
| Soil filler | 20-25 | 1.6 - 1.8 | 3.0 - 4.0 |
If the viscosity is too high, the material can be slightly heated (not higher than 30°C) or use a thinner recommended by the manufacturer. Never use acetone or 646 thinner to thin modern acrylic varnishes, this is guaranteed to lead to cloudiness and shagreen.
After mixing, the varnish should stand (exposure) for 10-15 minutes. During this time, air bubbles formed during stirring come out of the mixture. If the material is applied immediately, these bubbles will burst on the surface, leaving behind craters.
Use a thermometer to measure the temperature of the varnish, not just the air in the chamber. Cold varnish from a canister, once in a warm gun, will change its viscosity during operation.
Spray gun setup and test spray
Before adding beauty to your car, you need to adjust the torch on a test surface (cardboard or film). Adjustment is carried out by three screws: material supply, torch width and air pressure.
First open the air supply to maximum, then adjust the material supply. A properly tuned torch should have the shape of an elongated oval with clear boundaries and uniform saturation in the center and edges. If the edges are torn or the material spits, it means the pressure is too low or the nozzle is clogged.
Application technique also affects shagreen. The distance from the nozzles to the surface should be constant - about 15-20 cm. When moving the hand, the distance should not “walk”. The speed of movement should be uniform: too fast - dust and shagreen will fall, too slow - drips.
☑️Torch customization
It is important to maintain the angle of the gun. It must be perpendicular to the surface at any point along the trajectory. If you "bow" with a gun, the thickness of the layer will be uneven, which will lead to different degrees of spreading and, as a result, to visible shagreen.
Application process: first and second layer
Applying varnish usually occurs in two stages. The first layer is the binder (or “fog”). It is applied thinly, from a distance of 25-30 cm, with quick movements. Its task is to create adhesion and a light base. Shagreen at this stage is acceptable and even necessary for adhesion of the second layer.
The second layer is the main (glossy). It is applied after drying between layers (usually 15-20 minutes, when the first layer becomes matte). The second layer is applied wet-on-wet, slowly, from a distance of 15 cm. It is this layer that should spread into the mirror. It is important not to overdo it here: one pass with 50% overlap.
If you see that the varnish has begun to “float” or a large shagreen has formed, do not try to immediately cover it with a third layer. Allow the material to dry completely, then sand the defect and reapply varnish. Overheating the surface with several layers can lead to boiling of the varnish.
⚠️ Attention: When applying the second layer, avoid stopping your hand over the part. Any pause will lead to local accumulation of material and the formation of a difficult-to-remove drip that will ruin the entire look.
For large parts, such as the hood or roof, it is recommended to use the technique of "cross-pollination" or dividing the part into zones in order to control the flow of the material while it is still moving.
Drying and finishing
Proper drying is critical to avoid shagreen. If you raise the temperature too sharply, the surface of the varnish will “seize,” trapping the solvent inside. This will cause a “boiling” effect or large shagreen. The drying mode should be smooth: first 10 minutes at 20°C, then rising to 60°C.
Infrared drying is good because it heats the material from the inside, promoting better spreading. However, you need to be careful with them: the distance and exposure time must be strictly observed. Halogen dryers heat up the surface, which can lead to crust formation.
If after drying you still find small shagreen marks (which often happens even with professionals), it can be removed by polishing. But this can only be done after the varnish has completely polymerized (usually after 24 hours or after forced drying at 60°C for an hour).
The secret to polishing shagreen
Use an abrasive paste with a grit of 1000-1500 grit to remove shagreen, and then go to 3000 grit for a gloss effect. Do not start right away with a fine abrasive - you will only smear the relief.
Polishing removes the top micron layer, leveling the surface. After polishing, it is necessary to apply a protective wax or ceramic composition to fill micro-scratches from the polishing pad.
Common errors and ways to resolve them
One of the most common mistakes is saving on materials. Cheap varnishes have poor self-flowing properties. Even with perfect technique, they can produce fine shagreen. Professional varnishes marked “Self-leveling” are more forgiving of mistakes.
The second mistake is working in a dusty room or draft. Dust particles, settling on fresh varnish, create centers of tension around which the material does not spread, forming craters or tubercles. A draft accelerates the evaporation of the solvent, fixing the shagreen.
The third mistake is the painter's dirty clothes. Lint from overalls, hairs from dust brushes - all these are the enemies of smooth varnish. Use only high quality adhesive wipes immediately before application.
The main secret of the absence of shagreen is the balance between the viscosity of the material, air pressure and the speed of hand movement. Changing one parameter requires adjusting the others.
If shagreen has already appeared and it is strong (“orange peel”), polishing may not help, since you will have to remove too much varnish, right down to the base. In this case, the only way out is to carefully sand the entire part down to the base and re-varnish.
Is it possible to remove shagreen by polishing if it is very strong?
If the shagreen is deep, polishing will require removing a large layer of varnish (up to 15-20 microns). This is dangerous, as you can rub the varnish down to the base, especially on sharp edges of the body. In such cases, it is safer to repaint the part.
What is the ideal temperature for applying varnish?
The ideal air and surface temperature is +20...+22°C. At temperatures below +15°C the varnish becomes viscous and does not spread well. Above +25°C the solvent evaporates too quickly, fixing the shagreen.
Do I need to warm up the polish before applying?
Yes, if the varnish is cold (for example, brought from a winter warehouse), it must be kept in a warm room for at least 2-3 hours. Cold material has high viscosity and is guaranteed to produce shagreen.
Why did the varnish become shagreen even though I did everything according to the instructions?
The problem may be with the compressor. If it does not produce the required volume of air (performance) or the pressure “jumps” due to a small receiver, the torch will be unstable. Also check the hoses - a hose that is too long or thin "chokes" the air flow.