Painting a car or any other surface with a pneumatic spray gun is a process where the result depends 80% on the correct preparation of the paint material. Viscosity mixture is a critical parameter that determines the quality of application, the absence of smudges and uniformity of color. If the material is too thick, it will look like shagreen, and if it is too liquid, drips and low coverage are inevitable.
Many DIYers make the mistake of relying solely on the eye or following the instructions in the fine print on the cans, which are often advisory in nature. Real viscosity depends on the temperature in the paint box, air humidity and the specific sprayer model. Understanding the physical processes of mixing components will help you avoid costly rework and waste.
In this article we will look in detail at how to calculate the proportions of paint for a spray gun, what solvents to use for different types of enamels, and how to set up equipment for a specific mixture. Properly prepared material is the key to professional gloss and durability of the coating.
Why viscosity is more important than proportions
You'll often hear the phrase "divide 2 to 1," but that's just a starting point. Actual viscosity material is measured in seconds using a viscometer (DIN4 funnel) and can vary depending on temperature. A cold solvent will make the mixture thicker, and a warm solvent will make it thinner, which will require adjustment of the proportions.
Too high a viscosity leads to pneumatic spray gun cannot effectively break the material into microscopic droplets. As a result, large shagreen is formed on the surface, which will take a long time to sand. In addition, thick paint may not have time to spread, leaving visible transition boundaries.
On the other hand, excessively liquid material will result in the formation of “craters” and drips, especially on vertical surfaces. Solvent in this case, it evaporates too quickly, preventing the film from forming correctly, which can cause dullness or clouding of the varnish.
⚠️ Attention: Never add solvent “by eye” to a material already mixed with a hardener. This disrupts the chemical polymerization reaction and the coating may never reach full hardness or may lose its shine over time.
For precise control, use a viscometer. The standard viscosity for applying base is 14-18 seconds, and for varnish and primer - 18-22 seconds at a temperature of 20°C. This universal range, at which most modern HVLP spray guns operate at their maximum efficiency.
Types of paints and their mixing features
Different types of automotive enamels require an individual approach to preparation. Acrylic enamels (2K) are usually mixed with hardener in a ratio of 2:1 or 4:1, after which solvent (10-20%) is added. Here it is important to strictly observe the dosage of the hardener, since it is this that triggers the hardening reaction.
Basic paints (metallic, mother-of-pearl) do not contain a hardener and dry solely due to the evaporation of the solvent. They are usually diluted 1:1 or 2:1 depending on the manufacturer. The peculiarity of the base is that it shrinks as it dries, so the number of layers and their thickness are critical to the final color.
Filler primers often have a thicker consistency and require more solvent for the spray gun to function properly. Proportions can reach 4:1:1 (base:hardener:solvent). Using inappropriate solvent may result in poor adhesion or lifting of the old coating.
- 🎨 Acrylic 2K: Requires precise addition of hardener and is sensitive to the pot life of the mixture.
- 💧 Base (Metallic): It is diluted only with a solvent; the evaporation rate is important for grain placement.
- 🛡️ Ground: Allows greater dilution, but requires careful filtration.
Always read the Technical Data Sheet (TDS) for your specific product. Manufacturers PPG, Mobihel, Duxon or Vika may have their own recommendations that take precedence over general rules. Ignoring the manufacturer's instructions is the fastest way to failure.
Solvent selection: evaporation rate
Choosing the right solvent is half the success. It is classified by evaporation rate: fast, medium (normal) and slow. The choice depends on the ambient temperature and the area of the part to be painted. Temperature in the paint booth dictates what type of solvent must be used.
Fast solvent is used at low temperatures (up to 15-18°C) or for painting small parts. It evaporates quickly, preventing streaks, but can cause paint to “boil” or become dull at high temperatures. A slow solvent, on the contrary, is needed in hot weather (above 25°C) or when painting large surfaces, so that the paint has time to spread before drying.
⚠️ Attention: Using a quick thinner in hot weather is guaranteed to result in orange peel and dry spray defects that are nearly impossible to remove by polishing without repainting.
There is also a universal solvent that is suitable for working in the range of 18-25°C. However, for professional results, it is better to have a supply of all three types. Remember that 646th solvent is aggressive and not suitable for all modern materials, especially basecoats where it can cause swelling.
When mixing, always add the thinner last, after combining the base with the hardener (if required). Mix the mixture thoroughly, but try to do it slowly so as not to saturate the paint with air bubbles, which will then appear on the surface.
Table of mixing proportions of materials
Below are approximate proportions for the most common materials. However, as mentioned earlier, the instructions on the can always take precedence. The data in the table is valid for standard conditions (20°C).
| Material type | Proportion (Base: Hardener) | Adding solvent | Viscosity (DIN4) |
|---|---|---|---|
| Acrylic enamel 2K | 2 : 1 | 10-20% | 18-22 sec |
| Base paint (Metallic) | Not required | 50-100% (1:1 or 2:1) | 14-18 sec |
| Soil filler | 4 : 1 | 10-20% | 20-25 sec |
| Varnish (2K) | 2 : 1 | 5-10% (optional) | 18-22 sec |
| Liquid putty | 2-3% of volume | Not required (usually) | 30-40 sec |
The percentage of solvent added is calculated based on the volume of the finished mixture of base and hardener. For example, if you have 400 ml of a mixture of acrylic and hardener, then 10% is 40 ml of solvent. The use of graduated measuring cups greatly simplifies this process and increases accuracy preparations.
For varnishes, the addition of solvent is often not required, since they have an optimal viscosity “out of the can”. However, in hot weather or when using spray guns with a small nozzle (1.3 mm), it is allowed to add up to 5-10% fast or normal solvent to improve spreadability.
Setting the spray gun for viscosity
After preparing the mixture, you need to adjust the spray gun. Three main parameters: pressure, torch and material supply. The pressure at the outlet of the spray gun (not at the compressor pressure gauge!) for HVLP systems is 0.7-1.2 bar. For materials with high viscosity (soils), the pressure can be increased, and for the base, it can be decreased.
The width of the torch is adjusted depending on the size of the part. For large surfaces (hood, roof), open the torch completely. For narrow areas (pillars, sills), narrow the spray pattern to reduce material loss and fogging. The supply of material is regulated by a plug on the back of the spray gun.
- 🔧 Nozzles: For base - 1.3-1.4 mm, for acrylic and varnish - 1.3-1.5 mm, for primer - 1.5-1.8 mm.
- 💨 Pressure: Always check the pressure while the trigger is pulled, as static pressure does not reflect actual operating conditions.
- 🖌️ Test: Before painting, do a test pass on cardboard or a test plate.
If large droplets (“spitting”) are visible during the test, then the viscosity is too high or the pressure is low. If the material forms a dry mist and does not spread, add solvent or increase the flow. The ideal mark should be uniform, wet and glossy.
⚠️ Attention: Never exceed the maximum operating pressure specified in the spray gun data sheet. This will lead to increased fog formation, waste of paint and reduced quality of the coating due to rebound of the material from the surface.
☑️ Check before painting
Common mistakes and how to avoid them
One of the most common mistakes is using “old” solvent or acetone. Acetone it is too aggressive and evaporates quickly, which makes it unsuitable for thinning modern automotive enamels. This leads to clouding and poor adhesion.
The “lifetime” of a mixture is also often ignored. Acrylic paints with a hardener begin to thicken after 2-3 hours (depending on temperature). By trying to dilute thickened paint with a solvent, you only worsen the situation by disturbing the balance of the components. Prepare as much paint as you can process in 40-60 minutes.
Filtration is another critical step. Even new paint may contain skins or debris. The use of paint funnel filters (125-190 microns) is mandatory for each fill into the spray gun tank. This will save the nozzle from clogging, and the part from defects.
Don't forget about interlayer drying. If you apply the next coat of base too early, the solvent will “boil” underneath, causing bubbles. If you overexpose it, the interlayer adhesion will be disrupted. Follow the manufacturer's recommendations: usually 5-10 minutes at 20°C.
Use infrared drying or heat guns with caution. Local overheating can create a crust on the surface, under which the material remains liquid, leading to defects in the future.
Conclusion and final recommendations
The correct proportions of paint for a spray gun are a balance between theory and practice. The tabular data provides a basis, but the final viscosity should be adjusted to your cabinet conditions and equipment characteristics. Experiments using test plates before the main work saves time and nerves.
Remember that high-quality surface preparation and cleanliness in the work area are no less important than the paint itself. Dust that gets into fresh varnish will spoil the impression of even perfectly diluted enamel. Use tack cloths before each layer.
Investments in a quality viscometer, measuring cups and good solvents pay off from the first successfully painted part. Don’t skimp on materials, as remodeling always costs more than buying a quality one paint and varnish material.
Is it possible to mix paint from different manufacturers?
Strongly not recommended. The chemical composition of the base, hardeners and solvents for different brands (for example, Mobihel and Vika) may vary. This may cause the paint to not dry, curl up in the tank, or become discolored. Use components from the same system.
How long does paint dry before polishing?
Complete polymerization of acrylic and varnish coatings takes from 7 to 24 hours at room temperature. Polishing can only be done after complete hardening. If you start polishing too early, the abrasive will clog and marks will remain on the surface. When using IR drying, the time is reduced to 30-60 minutes.
How to wash the spray gun after work?
Immediately after work, flush the system with a solvent of the same speed as used for dilution (or universal). For acrylic and varnish, solvent 646 or 647 is suitable, but for the base it is better to use a special soft solvent so as not to damage the rubber seals of the spray gun.
Why does paint adhere to shagreen?
The main reasons: too high viscosity (not enough solvent), low air pressure, too long a distance from the spray gun to the part or rapid evaporation of the solvent (heat). Try adding 5-10% solvent and check the pressure settings.