High-quality application of a topcoat is not magic, but an exact science, where every gram of components matters. Many craftsmen, especially beginners, make the mistake of relying on the eye when mixing two-component compositions, which inevitably leads to shagreen, cloudiness or long drying. Car varnish requires a respectful attitude, since it is he who protects the paint from ultraviolet radiation and mechanical damage.
The process of preparing the mixture begins long before you open the lid of the hardener jar. It is necessary to take into account the temperature in the spray booth, air humidity and even the temperature of the varnish itself, which may be too cold after winter storage. Ignoring these factors negates all efforts to polish and prepare the body.
In this article we will analyze all the nuances of the chemistry of the process so that you can achieve a mirror shine the first time. You'll find out why HS and MS varnishes require a different approach, how to choose the right solvent and what to do if the mixture begins to thicken ahead of time. Proper preparation is 90% of the success of all painting work.
Chemistry of the process: types of varnishes and their features
Before you start mixing, it is important to understand what material you are working with. Modern systems are divided into several classes, and each of them dictates its own operating conditions. Most common two-component varnishes on a polyurethane base, which harden as a result of a chemical reaction between the base and the hardener.
There is a separation based on dry residue, which directly affects the number of layers and material consumption. HS varnishes (High Solid) contain a high dry matter content, which allows them to be applied in 1.5 layers, obtaining a thick, deep film. At the same time MS varnishes (Medium Solid) require the application of 2-3 layers, as they contain more volatile solvents that disappear during drying.
Standing apart are water-soluble compounds, which are becoming more popular due to environmental regulations, but they require specific equipment and strict microclimate control. For garage conditions and local repairs, classic solvent-soluble systems are most often used, where a special chemical composition plays the role of a diluent.
Selecting the right product depends on the area to be painted and the drying conditions. If you are working in a cold chamber without IR drying, you will need a varnish with accelerated polymerization, but you need to work with it very quickly, since pot life of the mixture will be minimal.
⚠️ Attention: Never mix varnishes from different manufacturers or different series in one jar. The chemical formulas of hardeners may differ, which will lead to wrinkling of the coating or its incomplete drying a week after painting.
Necessary tools and solvent selection
The quality of dilution depends not only on the components, but also on what you mix them with. Using dirty or unsuitable containers is a common cause of surface defects. You will need graduated measuring cups that allow you to accurately follow the proportions.
The key element here is solvent (thinner). It does not participate in the chemical curing reaction, but only regulates the viscosity for spraying. The wrong choice of solvent can lead to paint boiling or, conversely, to the coating taking too long to dry, on which dust will stick.
To work you will need the following set:
- 🧪 Measuring glass with divisions for precise dosing of components.
- 🥄 Wooden or plastic stirrer (metal may react).
- 🌡️ Thermometer for monitoring the temperature in the chamber and materials.
- 🎭 Painting filter (sieve) for filtering the mixture before pouring it into the spray gun.
Solvents are divided by evaporation rate: fast, normal and slow. The choice depends on the air temperature. If the room is hot, the fast solvent will evaporate while still in the torch, leaving shagreen. If it’s cold, slow will prevent the varnish from spreading, leaving drips.
Before mixing, warm the can of varnish to 20°C. Cold material has a high viscosity and is more difficult to mix evenly with the hardener, which can result in dull spots.
Mixing Ratios: Gold Standard
The most critical point is maintaining proportions. There is always a marking on the can, for example, 2:1 or 4:1. This is the ratio of the volume of varnish to the volume of hardener. An error of even 5-10% can radically change the properties of the final film.
If you add too much hardener, the coating will become brittle and may crack when first washed or exposed to sunlight. Too much hardener will also shorten the pot life of the mixture, and you risk throwing away half the can if it thickens in the glass.
Lack of hardener is an even more insidious mistake. The varnish can dry “touch” in an hour, but complete polymerization will never occur. This coating will be soft, stick to your hands and be easily scratched with a fingernail. Chemical reaction must go through to completion for the coating to last for years.
Typically, the proportion of thinner is 5-10% of the total volume of the mixture, but always refer to the technical data sheet (TDS) of the specific product for exact figures. Below is a table of standard ratios for various systems.
| System type | Proportion (Varnish: Hardener) | Adding solvent | Recommended Application |
|---|---|---|---|
| HS (High Solids) | 2 : 1 | 5-10% | Finish coating of the entire body, polishing |
| MS (Medium Solids) | 2 : 1 | 10-15% | Repair of parts, budget painting |
| VHS (Ultra High) | 3 : 1 | 0-5% | Getting maximum color depth |
| 1K (One-piece) | Ready to use | As needed | Plastic, small parts, no hardener required |
Technology for preparing the working mixture
The mixing process should take place in a clean area, away from open flames and dust. First, open the can of varnish and mix its contents thoroughly. During storage, heavy components may have settled to the bottom, and if they are not lifted, the upper part will be poor in resin.
Measure the required amount of base into a measuring cup. Then add hardener in strict proportions. Only after this, if necessary according to technology, is it added solvent. Mixing all three components at once can result in uneven distribution of the chemistry.
Stir the mixture vigorously with a stirrer for 2-3 minutes. It is important not to just swirl the liquid, but to work the stirrer along the walls and bottom, raising all the sediment. After this, let the mixture sit for 5-10 minutes. This is necessary to remove any air bubbles that were trapped during stirring.
Before pouring into the spray gun tank, be sure to strain the varnish through a filter funnel. Even a microscopic speck that gets into the mixture can become the center of defect crystallization or clog the spray nozzle.
☑️ Proper mixing of varnish
Adjusting viscosity and working with a spray gun
The finished mixture may have the ideal proportions, but the wrong viscosity for your spray bottle. A varnish that is too thick will form an “orange peel” that cannot be removed by polishing, while a varnish that is too thin will cause excessive drips.
To measure viscosity, professionals use a viscometer - a funnel with a calibrated hole. The flow time indicates whether more solvent needs to be added. For most HS varnishes, the optimal flow time is 18-22 seconds at 20°C.
If there is no viscometer, experienced craftsmen estimate the viscosity by eye by the stream flowing from the stirrer. It should be continuous, but not too heavy. However, this method is subjective and requires experience, so it is better for beginners to purchase a simple tool.
The spray gun setting also depends on the viscosity:
- 💨 Inlet pressure is usually 2.0-2.5 atmospheres for HVLP atomizers.
- 🔧 The torch opens to its maximum to create a uniform stripe.
- 🎚️ The needle stroke regulates the supply of material, which is more important than pressure.
⚠️ Attention: Do not try to dilute thickened varnish (which has begun to set) with a new portion of solvent. The chemical reaction has already started, and adding more chemicals will not reverse the process. This coating will have low gloss and poor adhesion.
Common errors and ways to resolve them
Even knowing the theory, it is easy to make mistakes in practice. One of the most common problems is “boiling” of the varnish. This occurs when the solvent is chosen too fast for the current temperature, or the layer is applied too thickly. The solvent does not have time to escape and breaks the film with bubbles.
Dullness or “blue discoloration” of the varnish often indicates moisture penetration. This can happen if the compressor is not drained of water, or if painting is carried out in a very damp room. Moisture condenses on fresh varnish, making it cloudy.
Poor flow (shagreen) can be caused by several factors:
- ❌ Insufficient air pressure in the spray gun.
- ❌ The distance from the spray gun to the surface is too large.
- ❌ High viscosity of the mixture (little solvent).
- ❌ High temperature in the chamber, which causes the varnish to dry on the fly.
If you notice that the varnish has begun to thicken in the glass faster than the allotted time (pot life), immediately stop working with this portion. Attempting to dilute it and continue application will result in failure. It's better to lose a little material than to remake the entire part.
What is Pot Life?
This is the period during which the varnish mixed with the hardener retains its working properties. Usually this is 2-4 hours at 20°C. After this time, the varnish thickens and becomes unusable, even if solvent is added.
Safety and waste disposal
Working with automotive varnishes and solvents requires strict adherence to safety measures. Solvent vapors are toxic and heavier than air, so they accumulate at the bottom of the room. Without quality ventilation, you can get seriously poisoned or lose consciousness.
Be sure to use a respirator with carbon filters of protection class A (organic vapors). Ordinary medical masks or dust petals are absolutely useless against chemical fumes. Eye protection and solvent-resistant gloves are also required.
The remaining mixture should not be poured down the drain or onto the ground. Cured varnish (together with cans and rags) must be taken to special hazardous waste disposal points. Remember that rags soaked in drying oil or varnish may spontaneously ignite, so store them in metal containers with water.
Compliance with proportions and temperature conditions is more important than the brand of varnish. A cheap varnish, diluted according to the instructions, will give a better result than an expensive one, mixed “by eye”.
Questions and answers (FAQ)
Is it possible to thin the varnish if it has thickened in the jar before mixing?
If the varnish has thickened in a closed factory container due to long storage, you can try to revive it. Add 5-10% of the appropriate solvent and mix thoroughly with a drill and attachment. However, if lumps or “jelly” have formed in the varnish, it can no longer be saved - just throw it away.
What is the difference between a hardener and a solvent?
A hardener is a chemically active component that reacts with the base of the varnish, causing it to harden. The solvent is an inert liquid that only makes the varnish liquid for application and completely evaporates without remaining in the film.
How long does the polish take to dry after application?
Touch drying time is 30-60 minutes at 20°C. Complete polymerization (hardening) takes from 7 to 14 days. During this period, it is not recommended to wash the car with aggressive chemicals or polish with abrasives.
Why did the varnish drip after application?
Drips occur due to the varnish being too thin (too much solvent), the layer applied too thick, or the surface temperature being too low. It could also be due to the slow hardener in the cold chamber.