Incorrectly selected inlet pressure spray gun for applying varnish is the most common cause of the appearance of shagreen, “orange peel” or, conversely, streaks on a newly painted surface. If the air stream breaks the material too much, the varnish layer turns out matte and rough, and if the pressure is insufficient, the thick varnish simply does not spray, falling in heavy drops. It is the balance between the viscosity of the material, the nozzle diameter and air pressure that determines the final quality of the coating, so the technical parameters of the equipment cannot be ignored.
The choice of tool directly affects the consumption of expensive material and the time spent polishing the finished product. Professional painters know that there is no one-size-fits-all solution: base and varnish often require different settings or even different HVLP and LVLP systems. In this article we will analyze the key aspects of selecting equipment to eliminate defects when painting.
Criteria for choosing a spray gun for varnish materials
The main parameter to look at when purchasing is the type of spray system. For working with varnishes, especially two-component and thick ones, systems are considered the most suitable HVLP (High Volume Low Pressure) and LVLP (Low Volume Low Pressure). The former provide a high volume of air at low pressure, which allows up to 70% of the material to be transferred to the surface, minimizing fog. The latter require less air from the compressor, but also demonstrate excellent results when working with viscous compounds.
The diameter of the nozzle (nozzle) plays a critical role. To apply varnish, a nozzle size of 1.3 to 1.5 mm is usually required, depending on the recommendations of the chemical manufacturer. Using a hole that is too small will cause the material to dry out quickly while still in the air and create a dry shagreen, which is almost impossible to polish without removing the entire layer.
- 🎯 System HVLP Ideal for garage conditions and work with powerful compressors, providing a soft application.
- 🎯 System LVLP Suitable for weak compressors and requires less air, maintaining high flame quality.
- 🎯 System RP (High Pressure) is less economical and creates more fog, but applies material faster on large surfaces.
The quality of the case materials also matters. Nickel-plated brass tanks are easier to clean and resist the harsh solvents required for post-varnish cleaning. Aluminum analogues are cheaper, but wear out faster when in contact with certain types of chemicals.
Technical characteristics: nozzle, pressure and torch
Settings spray torch - This is the first step before starting work. The torch shape regulator, located on the top of the spray head, allows you to change the shape of the spot from round to wide fan-shaped. For varnishing large body parts, such as the hood or roof, you need the widest possible fan to ensure uniform overlap of the stripes.
The pressure should be adjusted directly at the inlet of the tool by holding the trigger. For most varnishes, the range from 2.0 to 2.5 atmospheres is considered optimal. Exceeding this value leads to over-spraying and overspray, and decreasing it leads to poor spreading.
⚠️ Attention: Before starting work, be sure to check the pressure with the nozzle open (by pressing the trigger), since the static pressure in the system does not reflect the actual operating parameters.
The nozzle size must match the viscosity of the varnish. If you are using a thick anti-gravel or ceramic varnish, you may need a nozzle of 1.5 mm or even 1.7 mm, while for standard acrylic compositions 1.3-1.4 mm is enough. A discrepancy between the diameter of the hole and the viscosity of the material is a guaranteed path to marriage.
How to check the build quality of a pistol
Disassemble the air cap and check the needle for bending. Make sure the O-rings are lubricated with a special solvent-resistant lubricant and are not scored. The needle play should be minimal.
Preparation of paint and varnish material and filtration
The quality of the finishing coating depends 50% on surface preparation and 50% on material preparation. Varnish, especially two-component varnish, requires thorough mixing with the hardener. Violation of the proportions specified in the technical sheet will either result in the coating not drying or becoming too brittle and cracking.
Filtration is a mandatory step that beginners often ignore. Even a new bucket of varnish may contain dust particles, clumps or “grains” of hardener. Passing this step through a funnel filter (usually 125 or 190 microns) will cause the nozzle to clog during operation or cause imperfections in the gloss.
The viscosity of the material must be adjusted to working viscosity using a solvent. The measurement is carried out with a viscometer - a special funnel with a calibrated hole. The flow time of the stream indicates whether the material is ready for spraying or whether it still needs to be diluted.
Use only solvents recommended by the varnish manufacturer. A fast solvent can cause the varnish to boil in the sun, while a slow solvent can lead to drips in cold weather.
Application technology and feed adjustment
The material feed is adjusted by a screw that limits the needle stroke. Varnish is usually set to medium or close to maximum to ensure a rich, saturated layer. Insufficient supply will result in “dry” varnish that cannot be polished to a deep gloss.
The technique of moving the painter's hand must be worked out in advance. The gun must be held strictly perpendicular to the surface at a distance of 15-20 cm. Movements are carried out with the entire body, and not just with the hand, which ensures a uniform speed of passage. The overlap of each subsequent stripe should be approximately 50%.
☑️ Check before starting varnishing
Varnish is usually applied in two or three layers. The first layer (first pass) is made thin, dusty, to create an adhesive base. The second layer is applied more generously until a characteristic gloss (wet effect) appears. If a third layer is required, it is applied after interlayer drying of the second.
It is important not to hold the torch at one point, especially at the end of the passage, to avoid drips. The brush should move at a constant speed, starting before the trigger is pressed and ending after the trigger is released, already outside the part.
Table of defects and methods for eliminating them
Even experienced craftsmen encounter defects, the causes of which often lie in equipment settings or environmental conditions. Below is a table that helps you quickly diagnose the problem based on the appearance of the coating.
| Defect | Probable Cause | Remedy |
|---|---|---|
| Shagreen (orange peel) | High viscosity, low pressure, fast drying | Add solvent, increase pressure, slow down drying |
| Drips (sagging) | High feed, short distance, slow pass | Reduce feed, increase distance, speed up movement |
| Matting (loss of gloss) | Too high pressure, dry application | Reduce pressure, increase material flow |
| Craters (fisheye) | Grease or silicone on the surface, moisture in the air | Sand, degrease, check compressor filters |
Defect analysis should be carried out immediately after drying. If the shagreen is small, it can be removed by polishing. Deep defects, such as large craters or heavy streaks, often require local or complete repainting of the element.
The main secret of an even varnish is stability. Stable hand speed, stable distance to the part and stable air pressure give predictable results.
Tool care and safety
Washing the spray gun immediately after work is the painter’s golden rule. The varnish that has dried inside turns into stone, and it is almost impossible to clean it out of the channels without disassembling it and soaking it in aggressive chemicals. Washing should be done with a solvent appropriate for the type of varnish.
When working with varnishes, it is necessary to use personal protective equipment. Vapors from solvents and hardeners are toxic and may cause poisoning or an allergic reaction. A respirator with class A carbon filters is a mandatory piece of equipment.
- 🛡️ Use nitrile gloves, as varnish is difficult to wash off the skin and can cause dermatitis.
- 🛡️ Provide good ventilation of the room or use a spray booth.
- 🛡️ Regularly lubricate the moving parts of the needle with special lubricant.
⚠️ Attention: Never leave varnish in the spray gun tank, even for a short time. A polymerization reaction may begin inside the tool, rendering it unsuitable for further use.
The instrument should be stored disassembled, with a clean and dry air head. It is better to lightly lubricate the needle with oil and insert it back, but do not tighten it tightly so as not to deform the seals.
Frequently asked questions (FAQ)
Can varnish be applied with a paint sprayer?
Technically it is possible if the diameter of the nozzle allows the varnish to pass through without diluting it to the state of water. However, nozzles of 1.3-1.4 mm are often used for paint, which may be small for thick varnishes. It is better to have a separate set of nozzles or a gun for varnish, so as not to wash the instrument between layers of base and varnish, risking introducing dirt.
How many coats of varnish should I apply?
Standard technology involves applying 2 full wet layers. The first layer is a thin, binding layer. The second is the main, glossy one. The third layer is rarely applied, only if a special depth is required or if the second layer lays unevenly, but this increases the risk of drips.
Why does the varnish become dull immediately after application?
This phenomenon is called "fogging" or "whitening". The main reason is moisture trapped in the air or condensation from the compressor. It may also be due to the solvent evaporating too quickly in a cold or damp room.
What pressure should I use for the varnish?
The optimal working pressure at the nozzle is 2.0–2.5 atmospheres for most acrylic varnishes. The exact value is always indicated in the technical data sheet of the specific product, since different chemical compositions require different spray conditions.