Proper preparation of paint and varnish material is the foundation of high-quality body repair. Many car enthusiasts and even novice painters often underestimate the importance of choosing solvent, relying on intuition or advice from the Internet. However, it is the chemical composition of the thinner that directly affects how the metallic will lay down, whether streaks will appear and whether the gloss of the coating will be maintained.
Unlike plain enamels, paints with a metallic effect contain aluminum powder or mica, which require special conditions for uniform distribution. If you use a solvent that is too aggressive or, conversely, too slow, metal particles may sink into the layer or, on the contrary, float to the surface, creating an “apple-colored” or spotty effect.
In this article, we will analyze in detail what types of solvents exist, how to select them for the temperature in the spray booth, and what proportions are optimal for different painting systems. Understanding these nuances will allow you to avoid costly alterations and obtain factory-quality coating.
Types of solvents and their chemical characteristics
All solvents for automotive paints are divided into several main groups depending on the evaporation rate. This is a key parameter that determines how quickly solvents will evaporate from the paint film. To work with metallics, compositions based on esters, ketones and aromatic hydrocarbons are most often used.
Fast solvents (Fast) evaporate instantly. They are designed to work in cold conditions or low humidity. Using them in the heat will cause the paint to dry while still in the spray gun, which will cause the effect of “shagreen skin” or “orange peel”.
Slow solvents (Slow) allow the paint to remain fluid longer. This gives the aluminum particles time to properly orient themselves in the layer, which is critical for an even distribution of metallic luster. However, when used in a cold room, they can cause smudges or “boiling” of the paint.
⚠️ Attention: Never use acetone or Galosh gasoline to dilute high-quality acrylic or base enamels. These aggressive liquids can curdle the binder, changing the color and structure of the coating.
There are also universal compositions that are suitable for average temperature conditions. They are the golden mean, but require precise adherence to application technology. The choice of a specific type depends on the conditions in which you plan to paint.
Dependence of choice on ambient temperature
The temperature in the paint booth or garage is the main dictator when choosing a thinner. The physical processes of evaporation are directly dependent on heat. If you make the wrong choice, no amount of skill will save the coating from defects.
At temperatures below +15°C it is necessary to use fast solvents. They provide instant adhesion of the layer, preventing dust settling and sagging. However, you need to be careful with metallics: drying too quickly will prevent the metal particles from aligning, and the color will “play” in spots.
In the range from +15°C to +25°C (standard room temperature), universal formulations are used. These are the most favorable operating conditions when the evaporation rate is balanced. In such conditions, the metallic paint goes on smoothly and the risk of defects is minimal.
If the temperature exceeds +25°C, switch to slow solvents. They compensate for the heat, allowing the paint to spread evenly. Trying to use a quick solvent in hot weather will result in instant drying of the surface and the formation of dull spots that cannot be removed by polishing.
Before mixing paint, be sure to measure the temperature of not only the air, but also the metal of the body itself. Cool metal will slow down evaporation, even if the garage is hot.
Mixing Ratios: Gold Standard
Maintaining proportions is not just a recommendation, but a strict technology requirement. For metallic base enamels (Base Coat), the ratio is usually 2 parts paint to 1 part solvent (2:1), but the exact numbers are always indicated on the can by the manufacturer.
Over-thinning (adding extra solvent) reduces hiding power. You will have to apply more coats to cover up the primer or old color, which increases material consumption and work time. In addition, paint that is too thin can cause sagging and loss of color saturation.
Insufficient dilution (thick paint) will result in shagreen and poor flow. The spray gun will “spit” and the surface will become rough. Metal particles in a thick emulsion will not be able to orient themselves correctly, which will spoil the visual effect.
☑️ Control of paint preparation
Use graduated measuring cups to ensure accurate dosage. The eye in this matter is the enemy of quality. Even a small percentage deviation can significantly change the viscosity.
Viscosity and drying time table
For professional quality control, it is not enough to simply mix the components. It is important to understand how the mixture behaves over time. Viscosity is the resistance of a liquid to flow, and for a spray gun it must be ideal.
Below is a table showing the approximate tack dry (when dust does not stick) and complete dry time for different types of solvents at +20°C.
| Solvent type | Temperature | Touch dry time | Time before applying varnish |
|---|---|---|---|
| Fast | up to +15°C | 5-10 minutes | 15-20 minutes |
| Universal (Normal) | +15...+25°C | 10-15 minutes | 20-30 minutes |
| Slow | above +25°C | 15-25 minutes | 30-45 minutes |
| Special (Metallic) | Any (control) | Depends on the base | Strictly according to instructions |
Please note that drying times are based on thin layers. If you put a fatty layer, the time will increase. Before application varnish the base must completely “catch its breath” so that the solvent does not swell the finish coat.
Violation of interlayer exposure is the main reason for clouding of the varnish and the appearance of bubbles after drying.
Application technology and spray gun operation
After you have decided how to dilute the metallic paint and have mixed the components, the application stage begins. Not only technique is important here, but also instrument settings. The pressure in the spray gun for base is usually lower than for varnish or primer.
The optimal pressure is 2-3 atmospheres at the nozzle exit. If the pressure is too high, the paint will ricochet off the surface, creating a mist and a dry, rough finish. Low pressure will result in large drops and leaks.
Metallic is applied in several stages. The first layer is the binder (fog). It goes on semi-dry to create adhesion. The second and third layers are already wetter, but not to the point of gloss. It is in the wet layer that aluminum particles float up and settle.
⚠️ Attention: Do not try to cover the surface with one pass “into a wet layer”. This is guaranteed to lead to strong smudges and uneven distribution of metal powder.
Hold the gun strictly perpendicular to the surface. The tip of the spray gun changes the distance to the part, which affects the width of the torch and the amount of paint falling. Movements should be smooth, overlapping the previous pass by 50%.
What is “appleness” and how to avoid it?
Bullseye (or banding) is a defect where light and dark stripes are visible on the surface. This occurs due to the uneven distribution of metal particles. Causes: Incorrect solvent, poor mixing, poor spray technique, or insufficient time between coats.
Common errors and coating defects
Even experienced craftsmen sometimes encounter problems. Understanding the causes of defects helps to quickly correct the situation. Most often, problems arise due to violations of preparation technology or the use of low-quality materials.
One of the common mistakes is using varnish thinner to dilute the base. Varnish thinner is too aggressive and can lift the previous layer or primer, as well as change the shade of the pigment. Always use solvents labeled “for basecoats.”
Another problem is moisture ingress. Water may be contained in the compressor or condensed from the air. When exposed to paint, it causes dullness and craters. Be sure to use dehumidifiers and drain the condensate from the receiver regularly.
Poor filtration is the path to marriage. Even a microscopic speck that gets into the spray gun tank can ruin the entire appearance of the part. Use quality funnel filters every time you pour paint.
Always check the compatibility of solvent and paint on a test plate or in an inconspicuous place before the actual work. The reaction may be unpredictable.
Can metallic paint be thinned with regular thinner 646 or 647?
Technically it is possible, but the result will be unpredictable. Solvent 646 is very aggressive and dries quickly, which is almost guaranteed to lead to apples and shagreen on metallics. Solvent 647 is a little softer, but also not ideal. For a high-quality result, it is better to buy a specialized solvent from the same company as the paint (for example, Mobihel, Brulex, Body).
How long can already diluted paint be stored?
It is not recommended to store diluted base enamel (metallic). The chemical reaction begins immediately after mixing with the solvent. The maximum shelf life of the mixture in an open container is 2-4 hours. After this, the paint begins to thicken and its properties change. It is better to prepare as much as you need for one work session.
Why does a metallic color appear lighter or darker after it dries?
This depends on the orientation of the metal particles. If the particles lie flat, they reflect more light and the color appears lighter. If they stand upright or chaotically, the color darkens. This is affected by viscosity, spray gun pressure and, most importantly, the rate of solvent evaporation.
Do I need to matte the base before applying varnish?
No, the base enamel (metallic) cannot be matted before varnishing! It should dry naturally. Matting will disrupt the structure of the layer and lead to clouding of the varnish. The varnish is applied directly to the dried base.
How to clean the spray gun after working with metallic?
Immediately after use, rinse the instrument with the same solvent that was used for dilution. Pass it through the system, disassemble the needle and nozzle, and wipe with a soft cloth. Dried metallic is very difficult to remove and can scratch the seals and disrupt the spray pattern the next time you paint.