Introduction: Why solvent selection is important for auto repair
When working with paints and varnishes in auto repair, even the little things determine the final result. One of these “invisible” helpers is solvents. 646 and 647. At first glance, they seem interchangeable: both are colorless, both smell pungent, and both dissolve paints. But in practice, a mistake in choice can cost drips on a fresh surface, uneven gloss or even damage to the base.
These solvents belong to the group universal volatile mixtures, but their formulas are optimized for different tasks. For example, 646th more often used for diluting nitro enamels and varnishes, and 647th — for cleaning instruments from acrylic compounds. The difference lies in the proportions of the components: toluene, butyl acetate, ethyl cellosolve and others. We will analyze these nuances further so that you can choose the right option for body painting, degreasing parts or washing the spray gun.
Important: both solvents are classified as 3rd hazard class (moderately hazardous substances). This means that you need to work with them in well ventilated area, with protective gloves and respirator - especially when spraying. But more about this in the section on safety precautions.
Ingredients: what's inside the bottle with numbers 646 and 647
Both solvents are mixtures of organic compounds, but their proportions and additives determine the key differences. It is based on three main components:
- 🧪 Toluene (up to 50% in 646 and up to 60% in 647) - provides high dissolving power for nitrocellulose and acrylic coatings.
- 🧪 Butyl acetate (10–15%) - is responsible for the evaporation rate and gloss of the final layer.
- 🧪 Ethyl cellosolve (8–12%) - improves the fluidity of paint and prevents the formation of smudges.
The main difference is in the additional additives. B solvent 646 contained ethanol (up to 7%), which speeds up drying, but may cause clouding of some varnishes. B 647th added instead acetone (up to 5%), which makes it more aggressive to greasy stains and old layers of paint. That's why 647th more often used for degreasing metal before painting, and 646th - for dilution of finished paints and varnishes.
One more nuance: in 647th sometimes occurs xylene (up to 3%), which increases the dissolving ability for epoxy and polyurethane compounds. This makes it more versatile, but also more toxic. If you are working with water-borne paints, both solvents will not work - you need special formulations based on glycols.
Comparison of properties: table of key parameters
To clearly see the difference, let’s summarize the main characteristics in a table. Pay attention to the parameters critical for auto repair: evaporation rate, compatibility with materials and toxicity.
| Parameter | Solvent 646 | Solvent 647 |
|---|---|---|
| Evaporation rate | Medium (3–5 minutes) | Quick (1–3 minutes) |
| Main Application | Dilution of nitro enamels, varnishes, primers | Cleaning tools, degreasing, removing old paint |
| Compatible with acrylic paints | Limited (possibly cloudy) | Good (thanks to acetone) |
| Toxicity (hazard class) | 3rd (moderate) | 3rd (higher due to acetone) |
| Price (market average, 1 l) | 120–180 ₽ | 150–220 ₽ |
Critical nuance: solvent 647 is not recommended for diluting polyurethane-based car varnishes - it can cause delamination of the coating. At the same time 646th better suited for working with nitrocellulose enamels, which are often used for the restoration of retro cars or motorcycles.
⚠️ Attention: Never mix solvents 646 and 647 in one container! Their components may react, which will lead to a change in the properties of the mixture and an unpredictable result when painting.
Where and when to use each solvent
The choice between 646 and 647 depends on type of work and material, with whom you interact. Here's a quick checklist:
I use nitro enamel or nitro varnish → 646
It is necessary to clean the spray gun from acrylic paint → 647
I degrease the metal before priming → 647
I dilute the leveling primer → 646
Removing old paint from plastic parts → 647 (with caution!) -->
For body repair 646th indispensable when working with:
- 🚗 Nitrocellulose enamels (for example, for painting parts of motorcycles or bicycles).
- 🎨 Car varnishes based on nitro compounds (check the label on the can!).
- 🔧 Primers for nitro paints (for example, GF-021).
A 647th performs better in the following scenarios:
- 🧴 Cleaning Tools after working with acrylic or alkyd paints.
- 🧽 Degreasing metal surfaces before applying anti-corrosion compounds.
- 🔄 Removing old paint from body parts (but not from plastic - it can corrode!).
If you are working with two-component auto enamels (for example, Vika or Mobihel), both solvents will not work - you need specialized thinners from the paint manufacturer. The same applies to water-borne coatingsthat require solvent-based butyl glycol.
Before using paint thinner, always do a test: mix a small amount on a separate surface and check for clouding or separation after 10 to 15 minutes.
Safety precautions: how to work without risk to health
Both solvents are flammable liquids (flash point ~20–25°C) and emit toxic fumes. If not handled correctly they can cause:
- 🤒 Poisoning (dizziness, nausea) when inhaling vapors.
- 🔥 Fire in contact with open flames or sparks.
- 🧊 Skin frostbite with prolonged contact (due to rapid evaporation).
Minimum set of protective equipment:
- 😷 Respirator with class filter A1 (for organic vapors).
- 🧤 Nitrile gloves (rubber ones dissolve!).
- 👓 Safety glasses (vapors irritate the mucous membranes of the eyes).
- 🌬️ Exhaust ventilation or working outdoors.
⚠️ Attention: Never store solvents in plastic drink bottles! Static electricity or sugar residue may cause a fire. Use only metal canisters or special containers marked “Flammable”.
If solvent comes into contact with skin:
- Immediately wash the contact area plenty of water (at least 10 minutes).
- Apply neutralizing cream (for example, "Protective" from "Clean Line").
- If there is redness or itching, consult a doctor - a chemical burn is possible.
Solvents should be stored in cool place (at a temperature not exceeding +25°C), away from direct sunlight and heat sources. Shelf life in original packaging - 12 months, after opening - no more 6 months (due to evaporation of volatile components).
Common mistakes and how to avoid them
Even experienced craftsmen sometimes make mistakes when working with solvents. Here are the most common:
- Using 647 to thin varnish.
Consequences: cloudiness or “orange peel” on the surface. Solution: Always check compatibility on test part.
- Storage in loosely closed containers.
Consequences: change in composition due to evaporation, loss of properties. Solution: Transfer leftovers into a smaller container to minimize exposure to air.
- Working without a respirator indoors.
Consequences: headache, nausea, risk of chronic poisoning. Solution: Use a hood or work outside.
Another typical problem is incorrect ratio of solvent and paint. For example, for nitro enamels it is usually sufficient 10–15% 646th on the volume of paint. Exceeding the proportion of solvent leads to:
- 🎨 Reduced coverage (more layers will have to be applied).
- ⏳ Increased drying time (due to excess volatile components).
- 💧 Formation of drips on vertical surfaces.
What to do if the paint has already run?
If drips appear on a fresh layer (in the first 10–15 minutes), they can be carefully “stretched” with a clean brush dipped in solvent 646. If the paint has dried, you will have to sand the defect with sandpaper P1200–P1500 and apply a new layer.
When working with acrylic paints (for example, Duxone or Sikkens) it is better to use specialized thinners, since 646 and 647 can cause uneven drying or loss of shine. If there is no alternative, first test compatibility on an unnecessary part.
Alternatives: when 646 and 647 are not suitable
Despite their versatility, these solvents do not cover all tasks. In some cases, specialized compounds will be required:
| Problem | Recommended solvent | Reason |
|---|---|---|
| Thinning waterborne paints | Butyl glycol or propylene glycol | 646/647 do not mix with water |
| Working with epoxy primers | R-4 or R-40 | More aggressive formulation required |
| Cleaning plastic parts | Isopropyl alcohol (70%) | 647 corrodes plastic (especially ABS) |
| Painting "soft" plastics (bumpers) | Solvent for plastic (for example, APP W900) | 646/647 cause cracking |
For glass degreasing Before applying tinting or anti-rain coatings, it is better to use white spirit or isopropyl alcohol - they do not leave a greasy film, unlike 647. And for cleaning carburetors or fuel system will do solvent 648, which contains methyl ethyl ketone and copes better with oil deposits.
If you are working with powder paints (for example, for painting wheels), solvents are not needed at all - they are applied electrostatically and “baked” in an oven. But to clean equipment after such painting they use special washes based on glycols.
For modern automotive paints (acrylic, polyurethane), manufacturers always indicate the recommended thinner on the packaging. Ignoring these instructions will result in coating defects that cannot be corrected without repainting.
FAQ: answers to frequently asked questions
Can 646 and 647 be mixed to create a "universal" solvent?
No, it's dangerous! The components may enter into an unpredictable reaction, which will lead to a change in the properties of the mixture. For example, acetone from 647 will react with ethanol from 646, and the solubility will drop sharply. If you need a universal composition, it is better to buy solvent 648 — it combines the properties of both, but more balanced.
Why did white streaks appear after thinning the paint with 647?
This is a sign incompatibility solvent with paint pigments. Most often occurs with acrylic enamels containing titanium dioxide. Solution: Remove the layer by sanding and use the thinner recommended by the paint manufacturer (for example, P813-60 for acrylic).
Can 646 be used to remove paint from brake discs?
Absolutely not! Brake discs are made of cast iron or steel with a high carbon content, and toluene-based solvents can penetrate micropores in the metal and cause corrosion or deterioration of braking properties. To clean the brakes, use special cleaners (for example, LIQUI MOLY Bremsen-Reiniger).
How to properly dispose of leftover solvents?
Waste solvents are classified as hazardous waste class 3. They must not be poured down the drain or onto the ground! Take to a hazardous waste collection point or use for combustion in special furnaces (if the volume is more than 5 liters). Small quantities can be evaporated outdoors in a metal container, but this takes several days.
How to replace the 646 if it is not on sale?
As a last resort you can use solvent 649 (it is similar in composition, but with the addition of xylene) or a mixture of toluene (60%) + butyl acetate (30%) + ethylcellosolve (10%). However, for car repairs it is better not to experiment - buy the original 646 or a specialized thinner for your paint.