Why the correct proportion of solvent is critical to paint quality
Even the most expensive car enamel from PPG or Sikkens will turn into a useless slurry if the amount of solvent is incorrectly selected. Paint that is too thick will have an “orange peel” appearance, while paint that is too thin will run on vertical surfaces. But the problem is not only aesthetics: incorrect viscosity leads to peeling of the coating after 6–12 months, corrosion under the varnish and premature fading of the pigment.
Professionals in body shops use viscometers and strictly monitor the temperature in the paint booth, but in garage conditions they have to rely on experience and tabular data. In this article - exact proportions of solvent for acrylic, metallized and base paints for different tools (spray gun, roller, brush), taking into account air temperature. And also an analysis of typical mistakes that spoil the result even with high-quality materials.
Types of solvents for car enamels: which one to choose
There is no “universal” solvent - its composition depends on the type of paint, climatic conditions and even color. For example, for metallics slow evaporating solvents are required (so that the pigment does not settle at the bottom of the tank), and for dark acrylic enamels - with anti-spotting additives. Let's look at the main types:
- 🔹 White spirit and solvent - cheap, but aggressive. Suitable only for primers and alkyd enamels (e.g. ML-1110). They are not used for modern acrylic paints - they destroy the polymer base.
- 🔹 Solvent 646/647 - universal for nitro enamels and some acrylics. 646 evaporates faster, so it is added when painting in hot weather (+25°C and above).
- 🔹 Specialized solvents (PPG DT870, Sikkens Autoclear Thinner) - contain a balanced complex of esters and alcohols. Optimal for base paints and varnishes as they do not cause clouding.
- 🔹 "Slow" and "fast" solvents - differ in the rate of evaporation. At +15°C “slow” ones are used (for example, U-Pol Slow), at +30°C - “fast” (U-Pol Fast).
⚠️ Attention: Never mix different types of solvents (for example, 646 with white spirit) - this leads to paint separation and the formation of microbubbles. If you are not sure about the choice, use a solvent of the same brand as the paint (for example, for Mobihel — Mobihel Thinner).
Table of solvent proportions for different types of paints
Proportions are indicated in the format paint : solvent : hardener (if required). For base paints (metallic, pearl), no hardener is used! The data in the table is relevant for painting spray gun with a nozzle of 1.3–1.5 mm at a temperature of +20°C. For every ±5°C temperature deviation, adjust the amount of solvent by ±5–10%.
| Paint type | Proportion (paint: thinner: hardener) | Viscosity according to DIN 4 (seconds) | Notes |
|---|---|---|---|
| Acrylic enamel (1K) | 100 : 20–30 : 0 | 18–22 | No hardener. For Standox and Lesonal - up to 25% solvent. |
| Acrylic enamel (2K) | 100 : 10–20 : 50 (hardener) | 16–20 | The hardener is added first! The solvent comes last. |
| Base paint (metallic/pearl) | 100 : 50–70 : 0 | 14–16 | For PPG Deltabase - 60% solvent. Stir for at least 10 minutes. |
| Varnish (1K) | 100 : 20–30 : 0 | 20–24 | For Sikkens Autoclear — 25% solvent Sikkens Thinner. |
| Varnish (2K) | 100 : 10–15 : 50 (hardener) | 18–22 | Hardener U-Pol 2K mix at +20°C. At +10°C - increase the solvent to 20%. |
⚠️ Attention: For roller or brush The paint viscosity should be higher - reduce the amount of solvent by 10–15%. For example, for acrylic enamel under a roller, take a ratio of 100:10–15:50 (paint: solvent: hardener).
For base paints (metallic), 2–3 times more solvent is needed than for acrylic enamels due to the low pigment density.
How to measure paint viscosity without a viscometer
Viscometers are used in paint booths DIN 4 or Ford 4, but in garage conditions you can get by with improvised methods. The main thing is to achieve a consistency at which the paint:
- 🔸 Dripping from the stick uniform stream (does not drip or “tear”).
- 🔸 Leaves a thick mark on the glass 0.5–0.7 mm (check with a caliper).
- 🔸 When pouring from a can into the tank of a spray gun, it forms a “tail” long 15–20 cm.
To test, take a clean glass rod (or ruler) and dip it into the paint:
- Raise the stick vertically - the paint should flow off continuous stream for 3–4 seconds.
- If the stream stops earlier, add 5% solvent and mix.
- If the paint flows too quickly (less than 2 seconds), let it sit for 10 minutes: part of the solvent will evaporate.
What happens if the paint is too liquid?
Liquid paint forms streaks on vertical surfaces (doors, wings), and also causes the ground to “see through.” This is especially critical for dark colors—streaks will appear after drying.
5 mistakes when thinning paint that spoil the result
Even experienced painters sometimes make mistakes that only appear after drying. Here are the most common:
- Adding solvent to uncured paint. If you mix paint with a hardener and then add a solvent, partial polymerization will occur - microcracks will appear on the surface. Correct order: paint → solvent → hardener.
- Using expired solvent. Solvents based on acetone and xylene lose their properties 12 months after opening the can. Check the date on the packaging!
- Mix with a mixer at high speed. This leads to the formation of air bubbles, which remain in the form of craters after painting. The optimal mixer speed is 300–400 rpm.
- Ignoring room temperature. At +10°C the solvent evaporates 2 times slower than at +25°C. If you do not adjust the proportions, the paint will dry unevenly.
- Storing diluted paint for more than 4 hours. After mixing with the hardener, the paint begins to polymerize. If you leave it overnight, it will be unusable in the morning.
⚠️ Attention: If you paint plastic parts (bumper, mirrors), never use acetone-based solvents - they corrode the plastic. Take specialized formulations, for example, PPG DX330 or Spies Hecker Permahyd Hi-TEC Plastic Adhesion Promoter.
Determine the type of paint (acrylic, metallic, varnish)|Check the room temperature|Choose a solvent of the same brand as the paint|Add solvent to the paint (not vice versa!)|Mix with a mixer at low speed (300–400 rpm)|Check the viscosity with a stick or viscometer|Add hardener (if required) and mix for another 2–3 minutes-->
How to adjust proportions to air temperature
Temperature directly affects the rate of solvent evaporation. When painting in an unheated garage in winter or under the scorching sun in summer, standard proportions do not work. Use this adjustment table:
| Air temperature | Solvent adjustment | Solvent type | Drying time (relative to +20°C) |
|---|---|---|---|
| +5°C…+10°C | +15–20% | "Slow" (U-Pol Slow) | +50–70% |
| +15°C…+18°C | +5–10% | "Standard" (Sikkens Thinner) | +20–30% |
| +20°C…+25°C | 0% (standard) | "Standard" or "Universal" | 100% (base time) |
| +25°C…+30°C | −5–10% | "Fast" (U-Pol Fast) | −20–30% |
| +30°C and above | −15–20% | "Very fast" (PPG DT885) | −40–50% |
When painting in wet weather (more than 70%) add 5-10% solvent above the norm - moisture in the air slows down evaporation. Also keep in mind that metal surfaces (hood, roof) heat up more than plastic surfaces, so the paint on them dries faster.
If painting takes longer (for example, apply 3-4 layers), periodically check the viscosity of the paint in the spray gun tank. The solvent evaporates even after 15–20 minutes of work, and the paint may become too thick.
Features of paint thinning for different tools
The viscosity of paint depends not only on its type, but also on the tool you use. For example, for spray gun you need a thinner consistency than for rollerto avoid clogging the nozzle. Let's look at the nuances:
1. Spray gun (nozzle 1.3–1.5 mm)
- 🔸 Viscosity: 14–20 sec according to DIN 4.
- 🔸 Pressure: 2.0–2.5 atm.
- 🔸 For base paints (metallic) - maximum fluidity (up to 70% solvent).
2. Roller (for local repairs)
- 🔸 Viscosity: 22–26 sec according to DIN 4.
- 🔸 10–15% less solvent than for a spray gun.
- 🔸 Use foam roller with short pile (3–5 mm).
3. Brush (for small areas)
- 🔸 Viscosity: 26–30 sec according to DIN 4.
- 🔸 Solvent no more than 10-15% of the paint volume.
- 🔸 The brush must be natural bristles (for acrylic) or synthetic (for varnishes).
⚠️ Attention: When painting airbrush (nozzle 0.2–0.5 mm) the paint should be almost like water - viscosity 10–12 sec. Proportion: 1 part paint to 1.5–2 parts solvent. But this method is only suitable for artistic painting, and not for full coverage.
FAQ: Frequently asked questions about thinning car enamels
Is it possible to dilute paint with gasoline or kerosene?
No! Gasoline and kerosene contain impurities that do not evaporate completely. This leads to:
- 🔴 The varnish will turn yellow after 1-2 months.
- 🔴 Formation of a sticky layer on the surface (the paint does not polymerize completely).
- 🔴 Delamination of metallics - the pigment will settle to the bottom.
Exception - refined aviation kerosene (for example, KO-225), but it is also used only for washing instruments, and not for dilution.
What should I do if the paint is too thick and won't thin?
Possible causes and solutions:
- 🔹 The paint is old or stored at −10°C → Heat the jar in warm water (+30…+40°C) for 10–15 minutes, then stir.
- 🔹 The wrong solvent is being used → Replace with a specialized one (for example, for PPG - only PPG DT870).
- 🔹 The paint has started to polymerize → If more than 4 hours have passed since mixing with the hardener, the paint will have to be thrown away.
How to dilute paint for painting in hot weather (+30°C and above)?
At high temperature:
- Use a "fast" solvent (U-Pol Fast, PPG DT885).
- Reduce the amount of solvent by 10–15% relative to the standard proportion.
- Paint in the early morning or evening when the temperature is lower.
- Keep paint and tools in the shade - keep the cans away from heat.
⚠️ If the paint dries too quickly, marks will appear on the surface. microcracks (“craquelure”).
Is it possible to mix solvents of different brands?
Theoretically it is possible, but only if they are the same type (for example, two “slow” solvents for acrylic). It is strictly forbidden to mix:
- 🔴 Solvent based acetone (646) s alcohol (for varnishes).
- 🔴 “Fast” and “slow” solvents will disrupt the uniformity of drying.
- 🔴 Solvents for alkyd and acrylic paints.
It is best to always use the same brand of solvent as the paint.
How long can diluted paint be stored?
The lifespan of thinned paint depends on the type:
- 🔹 Without hardener (1K paints, base): up to 72 hours in an airtight container at +15…+20°C.
- 🔹 With hardener (2K paints, varnishes): no more than 4–6 hours. After this, irreversible polymerization begins.
⚠️ To extend the shelf life:
- 🔸 Pour paint into glass jar (plastic may react).
- 🔸 Add a layer on top solvent 1–2 mm thick - this will block the air supply.
- 🔸 Store in refrigerator (not in the freezer!) at +5...+10°C.