Solvents with markings 646, 647, 648, 649 and 650 - these are not just numbers on the label, but fundamentally different compositions, the choice of which determines the quality of painting, the durability of the coating and even the safety of work. In auto repair, these liquids are used to thin paints, degrease surfaces, clean tools, and even remove old paint layers. But why do some masters prefer solvent 646, while others recognize nothing but 650th? And why can an attempt to save money on the “wrong” composition result in drips, dullness, or even peeling paint?
In this article we will look at chemical composition, physical properties and practical nuances of application each solvent - from the most “aggressive” 646th to specialized 650th. You will find out which one is suitable for acrylic enamels, which one will save you from lumps in nitro paint, and which one absolutely cannot be used with plastic bumpers. And most importantly, how to avoid typical mistakes that even experienced painters sometimes make.
1. Composition and classification: what do the numbers 646–650 hide?
The marking of 64X series solvents dates back to the times of the USSR, when a unified designation system for paints and varnishes was developed. The numbers do not indicate the “prescription number”, but the functional group:
- 🧪 646 - a universal solvent for nitrocellulose paints and varnishes (the so-called “nitromaterials”).
- 🔬 647 - modification of 646 with the addition of ethyl cellosolve, which reduces toxicity but reduces dissolving ability.
- 🎨 648 — a specialized composition for diluting melamine alkyd and glyphthalic enamels (used in industrial painting).
- 🚗 649 — “automotive” solvent optimized for acrylic and alkyd car enamels.
- ♻️ 650 — “wash” for removing old paint and varnish coatings (do not confuse with thinners!).
All these solvents belong to the class volatile organic compounds (VOCs) and contain a mixture of aromatic hydrocarbons (toluene, xylene), alcohols (butanol, ethanol), ethers and ketones. However, their percentage is radically different, which determines the scope of application. For example, in solvent 646 the share of toluene can reach 50%, whereas in 649th it contains no more than 15%, but special plasticizers are added for working with acrylics.
2. Solvent 646: a “station wagon” with a catch
Solvent 646 - The most famous and Controversial (controversial) in the series. It is often called a “magic wand” for its ability to dissolve almost everything: from nitro varnishes to dried paint on brushes. However, this versatility has a downside.
Composition (GOST 18188-72):
- 🧪 Toluene - up to 50%
- 🧪 Ethyl cellosolve - up to 15%
- 🧪 Butyl acetate - up to 10%
- 🧪 Ethanol - up to 7%
- 🧪 Acetone - up to 7%
Thanks to the high toluene content 646th has maximum dissolving power among the entire series, but at the same time:
- ⚠️ Destroys plastics (including bumpers and moldings of modern cars).
- ⚠️ Causes clouding of acrylic varnishes if not diluted correctly.
- ⚠️ Has a strong toxic odor (work only with a respirator!).
⚠️ Attention: Never use solvent 646 for diluting acrylic car enamels of the brand Duxone or Sikkens - this will lead to delamination of the coating and the appearance of an “orange peel”. Suitable for these paints only 649th or specialized thinners from the manufacturer.
Where 646th truly irreplaceable:
- 🔧 Cleaning painting equipment (spray guns, hoses) from nitro enamels.
- 🎨 Dilution of nitro primers and nitro varnishes (for example, NTs-132).
- 🧴 Removing bitumen stains from the body (mixed with white spirit).
If you need to dilute nitro paint, but only 646 is at hand, add 10–15% to it solvent 647 - this will reduce aggressiveness and reduce the risk of coating clouding.
3. Solvent 647: a “mild alternative” to 646
Solvent 647 often referred to as a "lite version of the 646", and this is true - it is less aggressive, but also less versatile. Its key difference is replacing part of the toluene with ethylcellosolve, which reduces toxicity and reduces dissolving power.
Composition (TU 6-10-1726-87):
| Component | Content, % |
|---|---|
| Toluene | ≈30% |
| Ethyl cellosolve | ≈25% |
| Butyl acetate | ≈15% |
| Ethanol | ≈10% |
| Acetone | ≈5% |
Benefits 647th:
- 🌿 Less toxic than 646 (but a respirator is still required!).
- 🎨 Suitable for diluting nitro enamels if slow drying is required (for example, when painting large parts).
- 🔧 Safe for most plastics (except ABS).
Where to use:
- 🖌️ Dilution of nitro putties (NTs-008, NTs-009).
- 🧴 Cleaning brushes and rollers after working with nitro paints.
- 🚗 Degreasing metal surfaces before priming (but not before painting with acrylic!).
⚠️ Attention: Do not use solvent 647 for diluting acrylic primers HS (high solid) - this will lead to the formation of bubbles and poor adhesion. For such materials you need thinner for HS systems (for example, P850-60 from PPG).
4. Solvent 648: industry standard for melamines
Solvent 648 is a specialized composition for working with melamine alkyd and glypthal enamels, which are widely used in industrial painting (for example, for agricultural machinery, construction metal structures). It is used less often in car repair, but is indispensable when working with certain types of primers and old-style enamels.
Key components:
- 🧪 Xylene - up to 40%
- 🧪 Butyl alcohol - up to 30%
- 🧪 Ethylene glycol - up to 15%
Features 648th:
- 🔥 It has a high boiling point (up to 150°C), so it evaporates more slowly than 646 or 647.
- 🛡️ Forms a more durable film when drying, which improves the adhesion of subsequent layers.
- ⚠️ Incompatible with nitrocellulose materials (may cause paint to curl).
Where used in car repair:
- 🔧 Thinning primers based on melamine resins (for example, GF-021).
- 🚜 Painting of commercial vehicle parts (frames, truck bodies).
- 🛠️ Cleaning equipment after working with glyptal enamels.
Why is 648 not suitable for acrylic paints?
Acrylic enamels contain polymers that do not dissolve in xylene and butanol, the main components of 648. When mixed, the paint coagulates (curdles), which leads to the formation of lumps and an uneven coating.
5. Solvent 649: the “gold standard” for auto enamels
If you are looking for a solvent specifically for acrylic and alkyd car enamels, then 649th - it's your choice. It is designed taking into account the requirements of modern paints and varnishes and contains plasticizers, which prevent cracking of the coating.
Composition (TU 6-10-1143-87):
- 🧪 Xylene - up to 20%
- 🧪 Butyl acetate - up to 30%
- 🧪 Ethylene glycol - up to 10%
- 🧪 Special additives (plasticizers) - up to 5%
Benefits 649th:
- 🎨 Ideally compatible with acrylic paints (Duxone, Mobihel, Vika).
- 🛡️ Prevents the formation of “orange peel” and drips.
- 🌡️ Suitable for working at low temperatures (up to +5°C).
- 🔧 Safe for most automotive plastics (including ABS and polypropylene).
How to use it correctly:
- To dilute acrylic enamels, add 649th in a ratio of 1:1 (at +20°C) or 1:0.5 (at +10°C).
- To degrease the surface before painting, apply solvent to a lint-free cloth and wipe the part.
- To clean the spray gun after using acrylic paints, rinse it 649th, and then run clean through the system solvent 650.
Degrease the surface with 649-m|Check compatibility with paint (test on a small area)|Filter the solvent before mixing|Use a respirator with filter A1-->
6. Solvent 650: not for dilution, but for removal
Solvent 650 - that's a different story. Unlike previous lineups, it not intended for thinning paints, and is used exclusively as removing old paintwork. It is often called “liquid skin” for its ability to soften and remove even many years of layers of enamel.
Composition (TU 6-10-1379-86):
- 🧪 Paraffins - up to 30% (slow down evaporation).
- 🧪 Xylene - up to 25%
- 🧪 Toluene - up to 20%
- 🧪 Corrosion inhibitors - up to 5%
How it works 650th:
- Apply to the surface with a brush or spray.
- Leave for 10–30 minutes (depending on the thickness of the paint layer).
- The softened coating is removed with a spatula or brush.
- Residues are washed off with water or solvent 646.
Where it is used in car repair:
- 🚗 Removing old paint from body parts before restoration.
- 🔧 Cleaning welding seams from soil residues.
- 🛠️ Removing tinting from windows (mixed with ammonia).
⚠️ Attention: Solvent 650 contains paraffins, which leave a greasy film after evaporation. If you do not remove it before painting (for example, by washing the surface white spirit), the new coating will not stick!
650 is the only one in the series that cannot be used for thinning paints. Its task is to remove old coatings, not prepare new ones.
7. Comparison table: which solvent is for what
| Brand | Main purpose | Compatible paints | Safe for plastic | Toxicity |
|---|---|---|---|---|
| 646 | Dilution of nitro enamels, cleaning of instruments | NC, nitro varnishes, nitro primers | ❌ No | ⭐⭐⭐⭐⭐ (very high) |
| 647 | Gentle dilution of nitro enamels | NC, nitro putty | ⚠️ Partially (except ABS) | ⭐⭐⭐⭐ (high) |
| 648 | Dilution of melamine alkyd enamels | ML, GF, PF | ⚠️ Limited | ⭐⭐⭐ (average) |
| 649 | Dilution of acrylic and alkyd car enamels | Acrylic, alkyd, HS paints | ✅ Yes | ⭐⭐ (moderate) |
| 650 | Removing old paintwork | Not applicable | ❌ No | ⭐⭐⭐⭐ (high) |
8. Common mistakes and how to avoid them
Even experienced painters sometimes make mistakes when working with solvents. Here are the most common:
- Using 646 for acrylic paints.
Result: cloudiness, separation, “orange peel”.
Solution: replace with 649th or a specialized thinner from the paint manufacturer.
- Thinning paint by eye.
Result: uneven drying, drips, decreased gloss.
Solution: Use measuring containers and follow the manufacturer's recommendations (usually 10-20% solvent by volume of paint).
- Storing solvents in plastic bottles.
The result: the solvent “eats” the bottle, and microplastics and impurities get into it.
Solution: Store only in metal cans or glass containers with a tight lid.
- Ignoring temperature conditions.
Result: at low temperatures the paint does not flow well, at high temperatures it dries too quickly.
Solution: select a solvent based on the room temperature (for example, for +5°C, take “slow” 649th, for +30°C - “fast” thinner for hot climates).
Another common problem is buying fake solvents. There are often canisters on the market with the inscription “646”, inside of which diluted gasoline or acetone is spilled. How to recognize a fake:
- 🔍 Check the label: the original must indicate GOST or TU, the manufacturer’s address.
- 🧪 Smell: real 646th It smells strong, but not like gasoline.
- 💧 Color: high-quality solvent, transparent, without sediment.
FAQ: Frequently asked questions about solvents 646–650
Can solvents 646 and 649 be mixed?
No, it is not recommended to mix them. 646th contains aggressive components (toluene, acetone) that can damage the structure of acrylic paints diluted 649th. If you need to adjust the drying speed, it is better to add specialized retarders or accelerators from the same manufacturer as the paint.
What solvent is best for painting plastic bumpers?
For plastics (ABS, polypropylene) only suitable solvent 649 or specialized thinners for plastic (for example, PPG DX330). Be sure to apply before painting adhesive primer (for example, Plastic Primer from 3M), otherwise the paint will peel off over time.
How can I replace solvent 650 if it is not on sale?
Alternatives 650th:
- 🧴 Wash "Abro" or "Body 700" (for cars).
- 🧴 Homemade mixture: 60% white spirit + 30% acetone + 10% vegetable oil (to soften the paint).
- 🔥 Thermal method (hair dryer + spatula) - for small areas.
Important: after any washing, the surface must be rinsed white spirit and degrease antisilicon.
Why did streaks appear after thinning the paint on 646?
Causes of leaks:
- 💧 Too much solvent (recommended proportion exceeded).
- 🌡️ High temperature in the room (the solvent evaporates too quickly, not having time to spread).
- 🎨 Incorrect application technique (holding the spray gun too close or applying too thick a layer).
Solution: if streaks have already appeared, wait until completely dry, then sand the defect P1200–P1500 and apply a new coat with the correct viscosity (use a viscometer!).
Can solvents 646–650 be used to clean a carburetor?
No, these solvents are for cleaning carburetors and the fuel system don't fit. They leave an oily film and do not dissolve tar deposits. Use specialized cleaners:
- 🔧 ABRO Carb Cleaner (aerosol).
- 🔧 LIQUI MOLY Vergaser-Reiniger.
- 🔧 White spirit (for rough cleaning only, not for precision parts).