An incorrectly selected spray gun nozzle size, for example, using a 1.3 mm nozzle instead of 1.4 mm for a thick two-component varnish, leads to the formation of rough shagreen and the risk of the material “boiling” while still in the air cap. It is this technical nuance that most often becomes the cause of a defective coating when a master tries to save time on polishing or, conversely, ends up overusing an expensive composition due to excessive dilution. To achieve perfect, blemish-free gloss, it is critical to understand the viscosity of a particular varnish material and its correspondence to the nozzle passage opening.
In the process of selecting equipment, it is necessary to take into account not only the type of varnish, but also the temperature in the spray booth, as well as the pressure at the inlet to the tool. If you are using modern HS-varnish (High Solid) with high solids, trying to force it through a narrow 1.2mm hole will require excessive pressure, resulting in fog and material loss. In contrast, liquid varnish systems such as MS or water-soluble compositions require a smaller nozzle diameter to avoid the formation of drips and “orange peel”.
The main task of the painter before starting work is to diagnose the torch on the test surface, selecting the optimal combination of nozzle size and pressure. The wrong choice leads to the fact that the varnish lays down either too dry, forming matte spots, or too wet, flowing off vertical surfaces. Correctly selected air cap and nozzle ensure uniform distribution of particles, creating conditions for independent spreading (leveling) of the material after application.
Optimal nozzle size for different types of varnish
The choice of outlet diameter directly depends on the chemical composition and viscosity of the paint and varnish material. The standard recommendation for most modern high solids two-component varnishes (HS-lacquer) is to use a nozzle size of 1.3–1.4 mm. This range allows the material to exit at the desired pressure, providing good spray breakup without the need for excessive solvent dilution.
For less viscous materials such as medium-fill varnishes (MS) or some acrylic compounds, nozzles of 1.2–1.3 mm are often used. Using a hole that is too large, for example 1.5–1.7 mm, is justified only when working with very thick primary primers or special protective compounds, but not for finishing varnishing, where the thinness of the layer is important. The table below provides basic recommendations for matching materials and sizes.
| Material type | Recommended nozzle size (mm) | Features of application |
|---|---|---|
| Varnish HS (High Solid) | 1.3 – 1.4 | Requires good pressure for crushing |
| Varnish MS (Medium Solid) | 1.2 – 1.3 | Spreads easier, less risk of shagreen. |
| Acrylic varnish (1K) | 1.2 – 1.3 | Quick drying, requires work speed |
| Primer varnishes | 1.4 – 1.6 | High viscosity, thick layer |
It is worth noting that paint manufacturers often indicate the recommended nozzle size on the product data sheet (TDS). Ignoring this data may lead to disruption of the drying technology and reduced durability of the coating. If you are working with materials from different brands, a universal solution would be to have a replacement set of nozzles or use a spray gun with an adjustable outlet, although the latter are less common and more expensive.
The ideal compromise for a universal workshop is to purchase a spray gun with a 1.3 mm nozzle - it is suitable for 80% of modern varnishes and base enamels, allowing you not to change components with each change of material.
Influence of viscosity and temperature on nozzle selection
The viscosity of varnish is a variable value that directly depends on the ambient temperature and the material itself. As the temperature decreases, the viscosity increases and the varnish becomes thicker, which requires either increasing the temperature in the chamber or using a larger diameter nozzle. However, professionals prefer to regulate the temperature rather than change the nozzle, since increasing the hole can disrupt the geometry of the torch.
If the temperature in the spray booth is lower than recommended (usually +20°C), using a standard 1.3 mm nozzle for thick varnish may result in the material going on “dry” and not having time to spread. In such cases, some technicians temporarily increase the outlet pressure or add a little more solvent, but this is a risky path. A more competent approach is to warm up the material and air while maintaining the settings sprayer in normal mode.
⚠️ Attention: An attempt to compensate for low temperature by increasing pressure without changing the nozzle often leads to the formation of “fog” (dry spray), when small drops of varnish dry in the air before reaching the body, creating a rough surface.
Viscosity can be monitored using a viscometer, although experienced painters often rely on experience and the consistency of the flowing stream. For precision work, especially when using expensive premium varnishes, measuring the viscosity before pouring into the tank is a mandatory procedure. This allows you to accurately determine whether the current nozzle will fit or the material is too thick for the selected hole size.
Adjusting the pressure and shape of the torch
Even a perfectly selected nozzle will not give results without the correct air pressure setting. For a nozzle of 1.3–1.4 mm when applying varnish, the optimal pressure at the inlet to the spray gun is in the range of 2.0–2.5 bar (depending on the model of the tool). Insufficient pressure will not be able to qualitatively break the varnish into small particles, which will lead to large shagreen, and excess pressure will cause overspray and boiling.
The shape of the torch also plays a key role. To varnish large surfaces (hood, roof, doors), the torch is opened as wide as possible to ensure uniform overlap of the stripes. When working with complex reliefs or small parts, the torch can be narrowed, but you must ensure that the nozzle copes with the supply of material in a narrow cone without splashing. The adjustment of the material supply on the spray gun handle must be consistent with the width of the torch.
- 🎨 Wide flare is used for finishing coats on large areas for maximum smoothness.
- 🔧 The medium torch is suitable for applying the first layer of varnish.
- 🔍 A narrow torch is used only for local repairs or hard-to-reach places.
It is important to regularly check the cleanliness of the air cap. Clogging of the side channels responsible for the formation of the torch shape can distort the atomization even with the correct nozzle. Before starting work, always do a test shot on cardboard or film to make sure that the torch has the correct oval shape without “tails” or breaks.
☑️ Check before varnishing
Application technology: first and second layer
The varnishing process usually consists of applying a binder (fog) coat and a base wet coat. For the first coat, which applies thinly and dries quickly, the spray gun settings may be a little drier, but the nozzle size remains the same. The main task here is to create an adhesive base, so strong spreading is not required, and standard nozzle 1.3 mm copes with this task perfectly.
The second (and sometimes third) coat is applied “wet on wet” and requires maximum flow. At this point, it is critical that the viscosity of the varnish and the throughput of the nozzle allow the material to lay down in a smooth, mirror-like layer. If the nozzle is selected correctly, the varnish will spread on its own, eliminating spray marks. A mistake in sizing here can cost you some serious abrasive polishing.
Between layers it is necessary to maintain interlayer drying specified by the manufacturer. If you apply a second coat too early on an under-dried first coat, the solvent may “boil” inside, creating bubbles. If you are late, interlayer adhesion will be disrupted. In this case, the nozzle does not change; only the speed of hand movement and the distance to the surface are regulated.
⚠️ Attention: Never try to apply a thick wet layer of varnish through a narrow nozzle (1.0–1.2 mm), increasing the supply of material. This is guaranteed to lead to the formation of drips, since the material will not have time to be crushed by air.
Common errors and coating defects
One of the most common problems is shagreen (orange peel). If the varnish lays down with large shagreen and does not have time to spread, there may be several reasons: too rapid evaporation of the solvent, low temperature in the chamber, or, which is important for our topic, the nozzle is too small for a given viscosity. The material simply does not have time to level out before the surface layer polymerizes.
The other extreme is the formation of drips. This occurs when the nozzle is too large for the type of varnish or the supply pressure is too high. The varnish is applied too thickly; gravity causes it to flow down, disturbing the geometry of the part. In such cases, you have to wait for complete drying and sand off the defect, wasting time and materials.
- 🌫️ Dry spraying (matte spots) is a sign of high pressure or a small nozzle hole.
- 💧 Streaks and sagging are the result of a large nozzle, low viscosity or slow hand movement.
- 🕳️ Craters and fish eyes are often a result of pollution, but can be enhanced by the wrong torch.
Defect analysis helps to understand whether the right tool was selected. If shagreen is repeated time after time while following the technology, it is worth double-checking whether the nozzle matches the type of varnish used. It may be worth switching to a more modern spray gun model with improved air cap aerodynamics.
The secret of the masters
Many pros keep two guns - one with a 1.3 mm nozzle for varnish and base, and a second with 1.6–1.8 mm for primers and liquid putties, so as not to waste time replacing sets.
Nozzle care and spray gun maintenance
The durability and stability of the spray gun depend on proper care. Varnish that has hardened inside the nozzle channel can change the geometry of the outlet or completely clog it. After each use, it is necessary to thoroughly rinse the instrument with a special solvent, paying attention not only to the tank, but also to the air cap.
To clean the nozzle, it is strictly forbidden to use metal brushes or needles, which can scratch the inner walls. Even a microscopic scratch inside the nozzle will disrupt the laminar flow of the material, which will lead to beating of the torch and defects in the varnish. Use only soft brushes and special cleaning fluids.
Regularly lubricating the moving parts of the needle and checking the Teflon O-rings will also extend the life of the tool. If you notice that, with the same settings, the pressure “floats” or the torch has become unstable, the nozzle may be worn out or damaged and requires replacement. High quality stainless nozzle It lasts a long time, but it is sensitive to mechanical damage.
The quality of the finishing surface depends 80% on preparation, temperature and the selected nozzle, and only 20% on the skill of hand movement.
Is it possible to apply varnish with a 1.6 mm nozzle?
Technically it is possible if the varnish is very thick or it is a special primer-varnish, but for a topcoat this is a bad idea. A 1.6 mm nozzle will produce too coarse atomization grain, which will lead to rough shagreen that will have to be polished long and hard. For finishing it is better to use 1.3–1.4 mm.
Why does varnish boil in the nozzle?
This occurs when the air pressure is too high for a given hole size, or the temperature in the chamber is too high. The solvent begins to evaporate instantly, turning the varnish into dry dust as it exits the nozzle. You need to reduce the pressure or check the temperature.
How often do you need to change the nozzle on your spray gun?
With careful use and proper cleaning, a high-quality nozzle will last for years. It needs to be changed only if visible damage, corrosion appears, or if it no longer holds pressure and “spits.” Regular replacement is only required when using aggressive acidic soils.
Which nozzle is better for metallics: 1.3 or 1.4?
For most modern base metallic enamels, a 1.3 mm nozzle is optimal. It produces a smaller torch, which allows for better control over the placement of silver or mother-of-pearl. A 1.4mm nozzle can produce a larger droplet, which is sometimes useful for certain types of large droplets, but 1.3mm is considered a universal standard.
Does needle length affect the choice of nozzle?
Yes, the needle and nozzle are a pair. You cannot insert a needle from a 1.3 mm nozzle into a 1.6 mm body. They must be strictly from the same manufacturer and series, since the needle cone exactly matches the cone of the nozzle hole to ensure tightness when closed.