The appearance of shagreen, dullness or “orange peel” on a fresh paint coating most often indicates an incorrectly selected viscosity of the material or incorrect pressure at the outlet spray gun. If you see that the paint is applied unevenly, leaving unpainted spots or, conversely, drips, the first thing you need to do is check the ratio of solvent and paintwork material, and also make sure that the distance to the surface corresponds to the type of emulsion being sprayed. Incorrect adjustment torch leads to excess consumption of materials and defects, which will then have to be long and expensively eliminated by polishing or repainting.
Working effectively with a spray gun requires an understanding of the physical processes that occur when a spray of paint is broken up by air. The speed of passage of the material, the degree of its atomization (atomization) and the shape of the spot directly depend on three main parameters: air pressure, nozzle size and viscosity of the mixture. Ignoring the technical requirements of the paint manufacturer or using a low-quality compressor with low performance is guaranteed to lead to coating defects, since pressure pulsation will not allow the formation of a stable torch.
Before applying finishing coats, the operator must understand the basic principles of equipment and work area preparation. Critical understand that 80% of painting success depends on proper preparation and setup of the tool, and not just on the skill of hand movement. In this guide, we will analyze an algorithm of actions that will allow you to avoid common mistakes and get a professional result even in a garage.
Preparing the spray gun and selecting components
The foundation of high-quality painting is the correct choice of the tool itself and its components. Different types of work require different spray systems. The most common technologies today are HVLP (High Volume Low Pressure) and LVLP (Low Volume Low Pressure). The former require a large volume of air, but provide high material transfer, the latter are more economical and less demanding on compressor performance, which makes them ideal for small workshops.
The key element of the design is the nozzle (nozzle), the size of which is selected strictly according to the viscosity of the sprayed material. Using a hole that is too large for the base will result in overspray, and using a hole that is too small will result in the inability to push through thick varnish or primer. For water-based enamels, special nozzles with Teflon seals are often recommended, since water is more aggressive than solvents on metal parts.
- 🛠️ HVLP: High volume, low pressure - great paint transfer, but requires a powerful compressor.
- 💨 LVLP: Low volume, low pressure - saves air and material, forgives mistakes for beginners.
- 🔩 Nozzle material: Brass for budget models, stainless steel for professional work with aggressive chemicals.
Before assembly, all channels must be thoroughly cleaned of any remaining solvents. Even a microscopic speck of dust that gets into the paint supply channel can disrupt the geometry of the torch. Washing should be done with a specialized liquid, avoiding the use of metal brushes, which can scratch the inner surface of the nozzle and spoil the spray.
Always check the O-rings on the needle and cup before assembly. Dried or damaged gum will lead to air leaks and unstable operation of the spray gun.
Adjustment of pressure and torch shape
Setting up a spray gun is not a one-time task, but a process that may require adjustments depending on the temperature in the chamber and the type of layer being applied. Adjustment is carried out by three main screws located on the body of the instrument. The paint supply screw limits the needle stroke, the spray pattern screw changes the spray geometry, and the air supply screw regulates the inlet pressure.
To adjust the shape of the torch, direct the jet to a vertical test surface (cardboard or metal) from a distance of 15-20 cm. Briefly pressing the trigger should leave a clear imprint. If the spot has the shape of an elongated oval with clear boundaries, the setting is correct. If “horns” are visible at the edges or the spot is round, you need to turn the torch shape regulator.
⚠️ Attention: Never adjust the air screw while spraying the part. This will lead to a sharp rise in pressure and the formation of defects. Make all settings only on the test surface.
Air pressure is the most delicate parameter. Too much pressure will cause fogging and dry coating as the solvent will evaporate while still in the air. Too low will not be able to effectively break up the drops, which will lead to large shagreen. The optimal value is usually indicated by the paint manufacturer in the technical data sheet and is measured directly at the moment the trigger opens.
Mixture preparation and viscosity control
The viscosity of a paint and varnish material is a parameter that determines its fluidity. It depends on it how the paint will lie on the surface. Manufacturers indicate the recommended viscosity in seconds on a viscometer (usually DIN-4). For measurement, a special funnel is used, the time of flow of the jet from which is the desired value.
The process of preparing the mixture requires precision. The mixing proportions of the components (base, hardener, solvent) must be observed to the nearest gram. The use of measuring cups or electronic scales is mandatory. You cannot dilute professional car enamel “by eye”, as this will disrupt the chemical polymerization process and (can lead to) clouding or peeling.
| Material | Nozzle size (mm) | Pressure (Bar) | Viscosity (DIN-4, sec) |
|---|---|---|---|
| Soil filler | 1.7 – 2.0 | 2.5 – 3.5 | 18 – 22 |
| Base enamel | 1.3 – 1.4 | 2.0 – 2.5 | 16 – 18 |
| Acrylic varnish | 1.3 – 1.5 | 1.5 – 2.0 | 14 – 16 |
| Liquid putty | 2.0 – 3.0 | 3.5 – 4.5 | 25 – 30 |
The temperature of the mixture also plays a role. Cold paint is more viscous, and even adding solvent may not have the desired effect if the temperature in the chamber is low. The optimum material and surface temperature should be around 20°C. If the paint is too thick, it will not spread well, forming a rough shagreen.
Effect of temperature on viscosity
When the temperature increases by 10°C, the viscosity of most paints and varnishes decreases by approximately half. This means that in hot weather you need to use a less volatile (slow) solvent so that the paint does not have time to dry on the fly.
Application technique and hand movement
The correct position of the spray gun relative to the surface is the key to a uniform layer. The tool must be held strictly perpendicular to the plane of the part. Any tilt will result in more layering on one side than the other, causing unevenness or streaking.
The distance from the nozzle to the surface is usually 15-20 cm for enamels and 20-25 cm for varnishes. The movement of the hand should be smooth, carried out by the whole body, and not just the hand. The speed of movement must be constant: too slow movement will cause drips, too fast - dry coating.
- 🖌️ Overlap: Each new pass should overlap the previous one by 50-70%. This ensures uniformity and no streaks.
- ⏱️ Timing: Interlayer drying must be followed strictly according to the instructions. An under-dried layer may “boil” when applying the next one.
- 🔄 Trajectory: Start the movement before pressing the trigger and end after releasing it, so as not to “spit” on the edges of the part.
⚠️ Attention: When painting large surfaces (hood, roof), change your body position, but keep the tool parallel to the surface. Do not describe arc-shaped trajectories with your hand, keeping your elbow suspended.
The application of the first layer is often done “foggy” (spraying). This allows you to create an adhesive base and bind dust. Subsequent layers are laid wet, but without the formation of thick films, which can cause the formation of bubbles. It is important to feel the moment when the surface becomes glossy.
☑️ Checklist before painting
Elimination of paint defects
Even experienced painters encounter defects, but they know how to recognize and eliminate them. One of the most common problems is “orange peel” (shagreen). It occurs due to too high paint viscosity, incorrect pressure or rapid evaporation of the solvent. Light shagreen may require polishing to remove, while heavy shagreen will have to be sanded off and repainted.
Drips (sagging) are formed when applying too thick a layer, insufficient drying between layers or bringing the spray gun too close. If a drip is noticed immediately, you can try to pick it up with a soft brush or remove the excess with a stream of air, but most often you have to wait for it to dry completely and sand off the defect.
Dry coating (matte on gloss) occurs when drops of paint do not have time to spread before drying. This happens when there is high pressure, a long distance to the part, or using a solvent that is too fast in hot weather. This can only be corrected by completely repainting the element after sanding.
The main reason for 90% of defects is a violation of surface preparation technology and ignoring recommendations for interlayer drying, and not equipment malfunction.
Tool maintenance and cleaning
The service life of the spray gun and the quality of its work directly depend on how you wash it. Remains of paint frozen in the channels or on the needle may break off during the next job and ruin the coating if it gets under the stream. In addition, dried paint changes the geometry of the internal channels, disrupting the aerodynamics of the flow.
Flushing should begin immediately after completion of work. First, a solvent corresponding to the type of paint used is poured into the tank and blown through the system. Then the spray gun is disassembled, and all removable parts (nozzle, needle, air cap) are soaked in a special bath for cleaning.
Use soft brushes and brushes to clean the outer parts, but never scrub the inside of the nozzle with metal objects. After washing, all parts must be wiped dry and lubricated with special oil for spray guns before assembling for storage. This will prevent corrosion and jamming of moving parts.
How to properly store a spray gun?
The instrument should be stored disassembled or with the needle spring weakened so that it does not deform over time. It is best to place the disassembled kit in the original case, protecting the nozzle and needle from mechanical damage. Do not leave the spray gun with solvent inside, as harsh chemicals can corrode the seals.
Why does the spray gun spit?
If large drops fly from the spray bottle or it spits, check: 1) whether the air cap is screwed in tightly; 2) whether the paint on the end of the needle has dried; 3) is the paint level in the tank sufficient (for gravity models). A damaged needle seal may also be the cause.
What kind of compressor is needed for a spray gun?
The compressor capacity (l/min) should be 20-30% greater than the spray gun consumption. If the tool requires 300 l/min, the compressor should produce a minimum of 400-450 l/min. The volume of the receiver is also important: for painting a car, it is advisable to have at least 50-100 liters so that the pressure does not jump.