In the process of painting a car, degreasing surfaces or cleaning tools, the master inevitably faces the question of choosing a chemical. On store shelves you can see dozens of cans marked from P-4 to 650, and it is easy for an inexperienced car enthusiast to get confused in this digital code. The wrong choice of fluid can lead to paint defects such as shagreen, clouding or even peeling of the enamel.
The numbers in the name are not a random set of characters, but a strictly regulated code indicating the chemical composition and scope of the product. Understanding the numbering logic allows you to instantly determine whether a fluid is suitable for working with nitrocellulose varnishes or is intended for epoxy resins. In this article, we will analyze the notation system in detail so that you can confidently choose materials for your tasks.
The main difference lies in the rate of evaporation and the aggressiveness of the effect on the polymer paint matrix. Solvent must evaporate at a certain rate: too fast drying will lead to “boiling” of the paint, and too slow drying will lead to drips and dust adhesion. That is why there are dozens of types, each of which is tailored for a specific class of paints and varnishes.
Numbering logic: GOST vs TU
In the paint and varnish materials industry, there have historically been two main marking systems, which often confuse newcomers. The first one is classic solvents according to GOST, which are designated by the letter “P” and a number (for example, P-4, P-12). The second system is the so-called “numbered” solvents (646, 647, 650), which appeared later and are often produced according to technical specifications (TU).
The key difference lies in composition and versatility. Products with the letter “P” usually have a narrower specialization and a composition strictly regulated by GOST, where the exact percentages of components are specified. Numbered solvents such as R-646, were originally developed as universal mixtures for a wide range of tasks, which makes them more popular in garage repair, but also more aggressive.
It is important to understand that the number in the number does not always indicate the "power" or evaporation rate in a linear manner. For example, solvent 647 evaporates faster than 646, despite the proximity of the numbers, and has a slightly different balance of aromatic hydrocarbons. Therefore, choosing a material “by eye” or according to the principle “the higher the number, the better” is a gross mistake that can cost the quality of the painting.
⚠️ Attention: Never use high acetone solvents (such as 646) to thin water-based acrylic enamels or some plastics. This may lead to irreversible dissolution of the base layer or swelling of the surface.
Classic solvents of the “R” series (GOST)
The classic ruler, starting with the letter "P", is the de facto standard for many industrial and repair work. These liquids were created in Soviet times and are still produced according to strict standards. Solvent R-4 - one of the most popular representatives of this group, which is a mixture of butyl acetate, acetone and toluene. It is ideal for working with nitrocellulose varnishes and enamels.
Another important player is R-12, which contains freon and is used primarily for working with fluoroplastic and in refrigeration technology, but is less common in auto repair. Traditionally used for diluting glypthal and pentaphthalic paints, as well as bitumen varnishes. R-5. These materials, as a rule, are less aggressive to plastics than their serial counterparts, but require a more precise selection for the type of paint.
The main advantage of the “P” series is predictability. If the technical data sheet for automotive enamel indicates the use of P-4, you can be confident in the result, since the composition of this liquid is stable from year to year. However, it is worth remembering that they often have a stronger odor and less solvent power in relation to modern synthetic resins.
The secret of mixing P-4 and P-5
Some craftsmen experiment by mixing P-4 and P-5 in a 1:1 ratio to obtain a solvent with an average evaporation rate. This is only permissible for primers and non-critical coatings, but is strictly prohibited for metallic finishing layers, as the orientation of aluminum particles may be disrupted.
Universal number solvents (646, 647, 650)
A numbered group led by the legendary R-646, has become the standard for auto body repair due to its high solvent power. Unlike the classics, these are multicomponent mixtures, including alcohols, ketones, esters and aromatic hydrocarbons. Solvent 646 It is considered universal and is suitable for most nitro and acrylic materials, including varnishes and enamels.
If faster drying is required, for example when working in a cold environment or for intermediate layers, use R-647. It contains more fast-evaporating components (acetone and ethyl acetate), which reduces drying time between coats. However, its aggressiveness is higher, and the risk of paint “boiling” if applied incorrectly also increases.
For jobs that require slow evaporation and smooth spreading (for example, in hot weather or for large surfaces), use R-650. It contains fewer ketones and more slow-evaporating hydrocarbons, which provides excellent flow of varnish without shagreen. The choice between these numbers is essentially a choice of operating speed and environmental conditions.
When working with numbered solvents (646, 647), always check the reaction of the plastic in an inconspicuous area. High ketone content can instantly melt some types of polypropylene and ABS plastic.
Comparison table of characteristics
To systematize the information and help you quickly navigate the assortment, we have prepared a summary table of the main characteristics. It shows how products differ in evaporation rate and main purpose.
| Marking | Base type | Evaporation rate | Main Application |
|---|---|---|---|
| R-4 | Ketones, esters | Average | Nitro enamels, nitro varnishes |
| R-646 | Blend (alcohols, ketones) | Medium/High | Acrylic paints, varnishes, primers |
| R-647 | Ketone Focused Blend | High (fast) | Nitrocellulose enamels, quick drying |
| R-650 | Blend with emphasis on hydrocarbons | Low (slow) | Varnishes, enamels for smooth spreading |
The table shows that the difference between the numbers lies in the balance of components. R-646 occupies the golden mean, which made it the best-selling. However, for professional painting in a booth, specialized thinners (Hardener/Thinner) with manufacturer’s markings are often used (for example, Mipa, Mobihel, Brulex), which according to the number system can correspond to the 650th or 647th, but have a purer chemical formula without impurities.
The use of cheap commercial solvents for expensive two-component materials may be economically justified during surface preparation (degreasing), but is risky for the final layer. Chemical additives in cheap products can react with the hardener, changing the color or strength of the coating.
Specialized compounds and their features
In addition to universal brands, there are solvents for specific tasks. For example, R-1191 or R-1192 often used for working with epoxy resins and polyurethanes. They have a unique set of components capable of dissolving very resistant polymers that ordinary acetone does not “take.”
For cleaning bitumen stains or working with rubber mastics, use R-40 or R-400 (not to be confused with R-4!). These are heavy oil fractions that practically do not evaporate at room temperature and act by penetration and softening. You absolutely cannot use them to dilute paints - they will leave a greasy film that cannot be removed.
☑️ Checking the quality of the solvent
Also worth mentioning White spirit (Nefras-S4-155/200). Although it does not have a 600+ style number, it is also a solvent, but for oil paints and alkyd enamels. It is rarely used in body repair, mainly for degreasing metal surfaces before priming, as it leaves a minimal greasy trace compared to aggressive chemicals.
⚠️ Attention: Store solvents only in original, tightly closed containers. Upon contact with air, light fractions evaporate and the composition of the liquid changes - it becomes less effective and can ruin the paint.
Safety and storage rules
Working with solvents of any marking requires strict adherence to safety precautions. All of them are flammable liquids (flammable liquids) and form explosive vapors. Solvent vapors are heavier than air, so they accumulate in the lower part of the room (in inspection holes, basements), creating a risk of ignition from a spark even at a distance of several meters from an open can.
When using aggressive compounds such as R-646 or R-647, be sure to use personal respiratory protection. Conventional petal respirators do not retain organic solvent vapors—class A filters (organic gases) are required. Prolonged inhalation of ketone and toluene vapors causes headaches, nausea and damage to the central nervous system.
Store jars in a cool, well-ventilated place, away from heat sources and direct sunlight. Plastic containers (for example, from mineral water) are not suitable for storing solvents - aggressive chemicals can dissolve the plastic, which will lead to leakage and fire. Use only metal or special chemical-resistant plastic containers.
The right solvent is 50% of the success of painting. It affects spreading, drying time, shine and absence of defects, so saving on chemicals when working with expensive materials is unacceptable.
Is it possible to mix solvents of different brands, for example, 646 and 647?
Technically you can mix, but the result will be unpredictable. You will change the volatility balance and dissolving power of the mixture. For technical needs (washing tools), this is acceptable, but it is not recommended to use such “cocktails” for diluting paint, as this can lead to paintwork defects.
What is the difference between solvent and thinner?
The solvent converts the solid resin into a liquid state (a chemical reaction), and the thinner simply reduces the viscosity of the finished liquid paint without entering into an active reaction. In everyday life, these concepts are often confused, but for professional car enamel it is important to use exactly what is indicated in the instructions (often these are special thinners).
Why did streaks appear on the plastic after using the 646?
Solvent 646 is very aggressive and contains many ketones. If the plastic has not been prepared or the type of plastic is incompatible with ketones, partial dissolution of the surface occurs, which gives a “dull” or streaky effect. For plastics, use special degreasers (anti-silicones).
What solvent is best for removing old paint?
R-646, R-647 or special paint removers (gel) are well suited for removing old nitro enamels. Acetone is also effective, but it evaporates very quickly, making the process less effective. For epoxy and polyurethane coatings, ordinary “chemistry” may not cope - special aggressive strippers are needed.