Why is proper paint thinning the key to quality painting?
Painting a car with a spray gun requires not only the ability to hold the tool, but also an accurate understanding of the chemistry of paints and varnishes. Incorrect paint to solvent ratio can lead to defects: from “orange peel” to streaks that will ruin even the most expensive enamel. Professionals know: SATA RP, Iwata W-400 or budget DeVilbiss FLG-4 — any spray gun requires an individual approach to dilution.
In this article we will analyze not only the standard proportions for acrylic, metallic and base paints, but also the nuances that are not mentioned in the instructions. For example, why at temperatures below +15°C, even the “correct” ratio of 2:1 can result in a defect, or how the pressure at the gun inlet affects viscosity. The material is based on the experience of painters with 10 years of experience and data from technical passports PPG, DuPont and Mobihel.
Types of paints and their characteristics when diluted
Not all auto enamels are equally fluid. The main types of paints used in body repair require different approaches to dilution:
- 🔴 Acrylic enamels (1K/2K): The most common for local repairs. Can be diluted with universal solvents (e.g. PPG D8105), but two-component systems require special hardeners.
- ⚡ Metallics and pearls: They require precise adherence to viscosity - if there is an excess of solvent, the pigments settle and the coating loses its “depth effect”.
- 🖌️ Basecoat: Can only be diluted with solvents recommended by the manufacturer (e.g. DuPont 3607S for water-soluble bases).
- 🛡️ Varnishes (HS/MS): High-hardness (HS) are diluted with slow solvents, medium-hard (MS) - with universal ones.
A critical mistake for newbies is to use the same solvent for all types of paints. For example, a fast solvent Mobihel 630 Suitable for acrylic in hot weather, but will ruin metallic by causing "spotting". Always check the compatibility on the can label!
Table of dilution ratios for different paints
Below is a universal table of proportions that is relevant for most brands (PPG, Sikkens, Standox). However, remember: The exact details are always on the label of your paint.! Workshop temperature and humidity may require ±10% adjustment.
| Paint type | Paint:solvent ratio | Recommended solvent | Application temperature |
|---|---|---|---|
| Acrylic 1K (repair) | 1:0.5–1:1 | PPG D8105, Mobihel 630 (fast) | +18°C–+25°C |
| Acrylic 2K (with hardener) | 4:1:1 (paint:hardener:solvent) | Sikkens Autoclear LV | +20°C–+30°C |
| Metallic/pearl (basecoat) | 1:0.8–1:1.2 | DuPont 3607S (for water soluble) | +15°C–+25°C |
| Varnish HS (high hardness) | 2:1 (varnish:hardener) + 10% solvent | Standox Vario HS | +20°C–+35°C |
| Varnish MS (medium hard) | 2:1 + 5–20% solvent | PPG D8120 (universal) | +15°C–+30°C |
⚠️ Attention: When working with water-soluble paints (DuPont Cromax Pro, PPG Envirobase>) Never use xylene or toluene based solvents - this will cause the paint to curdle in the gun!
How to measure paint viscosity: tools and life hacks
Even the ideal ratio according to the table does not guarantee the correct viscosity. Professionals use:
- 📏 Viscometer DIN-4: Auto repair standard. The viscosity of acrylic should be 18–22 sec at +20°C, varnish – 20–24 sec.
- 💧 Glass test: Drop paint onto a vertical glass - if it flows faster than 30 cm in 3 seconds, add paint.
- 🎨 Test Spray: Set the spray gun to 2.0-2.5 bar and make a test pass on the cardboard.
Critical point: Viscosity depends on temperature! At +10°C the paint thickens by 20–30%. Use heating the jar in a water bath (max. +30°C) or special solvents for low temperatures (PPG D8102).
Check the temperature in the workshop|Use a viscometer for an accurate measurement|Filter the paint through a 125 micron mesh|Do a test spray on cardboard-->
Top 5 mistakes when thinning paint and how to avoid them
Even experienced painters sometimes make mistakes that spoil the result. Here are the most common:
- Using the wrong solvent. For example, Mobihel 640 (slow) for acrylic in the heat will lead to prolonged drying and dust on the surface.
- Ignoring Humidity. For humidity >70% add 5–10% antisilicone (PPG D8010) into the solvent.
- Unfiltered paint. Dust particles or dried lumps will clog the spray gun nozzle. Always filter through
mesh 125–190 microns. - Mixing with a mixer at high speed. This creates air bubbles, which will then appear as craters on the coating.
- Storing thinned paint >24 hours. The hardener in 2K systems begins to polymerize even in a jar.
If the paint in the gun begins to “shoot” with splashes, this is a sign of too high viscosity. Do not dilute it directly in the tank - pour it back into the jar, rinse the gun and prepare a new portion.
How does the spray gun affect the paint ratio?
The type and settings of the spray gun directly affect the required viscosity. For example:
- 🔫 HVLP (low pressure, high volume): Requires a thinner paint (16-20 sec viscosity) as the spray pattern is softer. Suitable for metallics.
- 💨 RP (reduced pressure): Universal for acrylic and varnish, viscosity 18–22 sec.
- 🔥 Conventional (high pressure): Can work with thicker paint (20-24 sec), but produces more fog.
It is also important to consider nozzle size:
1.2–1.3 mm— for base paints and varnishes;1.4–1.6 mm— for acrylic and primers;1.7–2.0 mm— for thick materials (for example, liquid putty).
⚠️ Attention: If you are using a spray gun with a nozzle 1.4 mm for metallic, but the paint is too thick, the metal pigments will not be evenly distributed - you will get “spotting” after varnishing.
Practical advice from professionals
How to dilute paint for “blending”?
For a smooth transition during local repairs, use the ratio 1:1.5 (paint: thinner) and add 5% retarder (Sikkens Retarder). Spray from a distance of 30–40 cm, gradually reducing the pressure from 2.5 to 1.8 bar. This will help avoid a sharp line between the old and new paint.
A few more professional life hacks:
- 🌡️ Working in hot weather (>30°C): Use slow solvents (PPG D8101) and reduce the ratio to 1:0.8 for acrylic.
- ❄️ Working in cold weather (<15°C): Heat the paint and solvent to +20°C, add 10% drying accelerator (DuPont 7337S).
- 🔄 Switching to water-soluble paints: Rinse the gun special cleaner (PPG DX330), since a regular solvent will not remove the aqueous base.
The ideal thinning is a balance between paint flow and drying speed. Always do a test spray on cardboard or an old part before working on the body!
FAQ: Frequently asked questions about paint thinning
Can paint be thinned with gasoline or white spirit?
No! Gasoline and white spirit are incompatible with car enamels. They contain impurities that will interfere with adhesion and lead to paint peeling. Use only special solvents for car paints (PPG, Mobihel, DuPont).
Why did streaks appear after painting?
Causes: paint too thin (ratio >1:1.5), low gun pressure (<1.8 bar) or spray distance too close (<15 cm). To fix: sand away drips P1200–P1500 and repaint with the correct viscosity.
How to thin aerosol spray paint?
To pour paint into a spray can, use the ratio 1:1 with fast solvent (Mobihel 630) and add 5% propellant (for example, PPG D8020). The pressure in the cylinder should be 2.5–3.0 bar.
What is the difference between solvents for 1K and 2K paints?
Solvents for 1K paints (for example, PPG D8105) simply liquefy the material. For 2K systems solvents compatible with the hardener are needed (Sikkens Autoclear LV), otherwise the polymerization reaction will go wrong and the coating will be brittle.
How to store thinned paint?
Thinned paint (especially 2K) cannot be stored for >24 hours - the hardener will begin to polymerize. Seal the jar tightly and refrigerate (+5°C) for a maximum of 12 hours. For long-term storage use undiluted paint.