Direct application acid soil to a section of the body that has been stripped down to bare metal is a critical step in preventing the development of under-film corrosion in the future. Phosphoric acid, which is part of the primer, reacts chemically with iron oxides, creating a durable passivating film that blocks the access of oxygen and moisture to the surface. Errors at this stage, such as incorrect mixing proportions of two-component compositions or ignoring the exposure time, can lead to peeling of the entire paintwork after a few months of using the car.
The main purpose of using this material is not to level the surface, but to ensure maximum adhesion subsequent layers and chemical protection of the metal. Unlike acrylic fillers, acid soil (wash primer) does not have filling properties and is applied in the thinnest layer, literally “rubbing” into the metal structure. Professional painters often call this process etching, since the active components of the composition literally burn through micropores, creating an ideal base for adhesion.
There are two main types of such primers: one-component in aerosol cans and two-component, requiring the addition of a hardener immediately before work. For local repairs of small rust spots or hard-to-reach places, aerosols are often used, while when completely repainting body parts or working with large areas, it is preferable to use two-component systems applied with a spray gun. It is important to understand that acidic primer cannot be sanded to the metal, as this will destroy the created protective layer.
Principle of operation and chemical composition of the primer
The fundamental component of most etching primers is phosphoric acid, which reacts with metal oxides, converting unstable rust compounds into stable phosphates. This process, known as passivation, stops the further spread of corrosion even at those microscopic points that may have gone unnoticed by visual inspection after mechanical stripping. Without such chemical treatment, the risk of rust showing up under a new coat of paint remains extremely high, especially in conditions of high humidity or exposure to road chemicals.
The second key element of the composition is polyvinyl butyral resins, which provide high adhesion to both the metal base and subsequently applied materials. It is the presence of these resins that allows a strong bond to be created between the inorganic metal and organic coatings such as epoxy primers or putties. However, it is worth considering that acidic soils They are highly hygroscopic, that is, the ability to absorb moisture from the air if they are not insulated immediately after drying.
- 🧪 Active acids ensure deep penetration and neutralization of oxide residues.
- 🛡️ Polymer resins form a barrier layer with excellent adhesion.
- ⏱️ Fast drying time allows you to start the next operations in 15-30 minutes.
⚠️ Attention: Acid soil is an aggressive chemical. If it comes into contact with skin or mucous membranes, it can cause a serious chemical burn. Work must be carried out only in a well-ventilated area using personal protective equipment.
When working with acidic soils in enclosed spaces, be sure to use a respirator with a carbon filter, as vapors from solvents and acids can cause poisoning or an allergic reaction.
Surface preparation and necessary tools
The quality of the result directly depends on how thoroughly the metal surface was prepared before applying the primer. The metal must be absolutely clean, dry and grease-free, so use degreaser (anti-silicone) is a mandatory procedure that cannot be ignored even on visually clean areas. Any remaining oil, silicone, wax or dirt will create a barrier that will prevent the acid from contacting the metal, resulting in adhesion defects.
For mechanical surface preparation, gradated abrasives are most often used. P80-P120, which allows you to remove the oxide film and create the necessary microrelief (risk) for better adhesion. If there are remnants of old paint or putty on the surface, they must be removed to bare metal in the repair area, since acid soil Designed for application specifically to metal, and not to organic coatings. In some cases, application to old paints and varnishes is allowed if they are firmly adherent and compatible, but classical technology requires working with “bare” metal.
☑️ Preparation for priming
The tools for application depend on the selected type of soil and the scale of the work. For minor repairs, aerosol cans are convenient and do not require additional equipment other than a protective mask and gloves. For professional use in service stations, pneumatic spray guns with a nozzle of 1.3-1.5 mm, compressors with adequate performance and moisture-oil separators are used that prevent water and oil from entering the paint torch.
Acid primer application technology
Application process two-component primer begins with thoroughly mixing the base and hardener in the proportions indicated by the manufacturer on the packaging, usually a 1:1 ratio by volume. After adding the hardener, the mixture must be vigorously mixed and left for some time (usually 5-10 minutes) so that the primary chemical reaction takes place and air bubbles come out of the mixture. The finished composition retains its properties (“vitality”) for a limited time, usually from 30 minutes to 2 hours, so you need to mix exactly as much material as you have time to use.
When using a spray gun, it is important to adjust the pressure at the outlet of the nozzle in the range of 2-3 atmospheres and apply the material in a thin, semi-wet layer. Acidic soil no need to fill until gloss or streaks form; its task is to cover the metal with the thinnest film, so excessive application will only worsen the adhesive properties and increase the drying time. The movements of the gun should be smooth and uniform, overlapping the previous pass by 50%.
| Parameter | Meaning/Description | Note |
|---|---|---|
| Outlet pressure | 2.0 - 3.0 atm | Depends on material viscosity |
| Nozzle diameter | 1.3 - 1.5 mm | Optimal for liquid soils |
| Number of layers | 1 (maximum 2) | Total thickness up to 10 microns |
| Interlayer drying | 10 - 15 minutes | At +20°C |
The nuances of working in the cold season
At temperatures below +15°C, the polymerization time of acidic primers increases significantly, and the risk of moisture condensation on the metal surface increases. It is recommended to use infrared dryers to speed up the process or heat the room.
If you are using an aerosol can, shake it vigorously for 2-3 minutes before starting to mix the contents and activate the mixing ball. Application is carried out from a distance of 20-25 cm from the surface with short movements, avoiding holding the sprayer for a long time at one point. It is important not to try to cover the metal “tightly” the first time; it is better to apply several thin layers with intermediate drying.
Drying time and temperature conditions
Drying time acid soil is a critical parameter that determines the quality of further work. At a standard ambient temperature of +20°C and normal humidity, the material dries “dust free” in 5-10 minutes, and is fully ready for applying the next layer (for example, epoxy primer or putty) in 30-60 minutes. However, these indicators may vary depending on the manufacturer, the thickness of the applied layer and the specific climatic conditions in the spray booth.
Forced drying using infrared emitters or hot air can significantly reduce waiting time. At a temperature of +60°C, the polymerization process takes only 10-15 minutes, which is especially important in the conditions of continuous repairs at service stations. However, it is important not to overheat the surface immediately after application, as a sudden increase in temperature can cause the solvent to boil and cause pores or bubbles to form in the primer layer.
- 🌡️ Natural drying at +20°C: 30-60 minutes before covering.
- 🔥 IR drying at +60°C: 10-15 minutes until fully cooked.
- ❄️ At temperatures below +10°C, drying may take several hours or not occur at all.
⚠️ Attention: Do not leave a surface primed with acid primer open for a long time (more than 24 hours) without covering it with an insulating layer. The hygroscopicity of the material can lead to moisture saturation and loss of protective properties.
Compatibility with other materials and overcoating
One of the most important issues when using acid soil is its compatibility with subsequent coating materials. The classic repair scheme involves applying a layer over a dried acid primer epoxy primer, which acts as an insulator and equalizer. Epoxy primer creates an airtight film that protects the acid layer from moisture and provides an excellent base for acrylic putties or finishing paints.
There is a common misconception that any putty can be applied directly to acidic primer. In fact, polyester putties contain styrene, which can react with acidic soil components, causing it to swell or lose adhesion. Therefore, the rule "acid - epoxy - putty" is the most reliable and time-tested. If the technology requires applying putty directly to the etching primer, you need to make sure that the putty manufacturer guarantees such compatibility.
Finishing paints such as acrylic enamels, base paints and varnishes also require acid layer isolation. Direct application of paint to wash primer This is only possible if specifically stated in the technical documentation for the specific product, but in most cases an intermediate coat of acrylic or epoxy filler primer is required to create a smooth surface.
Acid primer is only the first step in a multi-stage body protection system. Without subsequent covering with insulating layers (epoxy, filler), its protective properties are lost, and the metal again becomes vulnerable to corrosion.
Common mistakes and safety precautions
The most common mistake when working with etching primers is violating the mixing proportions of the components or using an expired hardener. This leads to the fact that the chemical passivation reaction either does not start at all or does not proceed completely, leaving the metal unprotected. It is also common to make the mistake of applying too thick a layer in the hope of “protecting it for sure,” which paradoxically reduces adhesion and increases the risk of peeling off the entire paintwork pie.
Safety issues when working with acid compounds come first, since vapors of phosphoric acid and organic solvents are toxic. It is necessary to work in a respirator with appropriate filters, safety glasses and gloves. Contact of soil with exposed skin requires immediate rinsing with plenty of water, and if it gets into the eyes, seek immediate medical attention.
- 🚫 Application to wet or greasy metal (no degreasing).
- 🚫 An attempt to grind acidic soil “to zero” to metal.
- 🚫 Ignoring exposure time before applying the next layer.
Is it possible to apply acid primer to galvanized metal?
Yes, it is possible and even necessary. Acid primers work well on galvanized surfaces, galvanized zinc and aluminum, providing reliable adhesion that is difficult to achieve with conventional primers. However, the layer must be very thin.
Do I need to sand acid primer before applying epoxy primer?
In most cases, sanding is not required if the surface has been prepared correctly and the primer layer is thin. It is enough to wait for drying time and degrease the surface before applying epoxy primer. Sand only if there are application defects (dust, shagrel).
What is the difference between acid primer and epoxy primer?
Acid wash primer chemically etches the metal and does not create a thick film; it serves for adhesion and primary protection. Epoxy primer creates a mechanical barrier, isolates from moisture and has high anti-corrosion properties, but does not etch the metal as actively.
How long does it take for acidic soil to dry at low temperatures?
At temperatures below +15°C, the drying process may take 2-3 times longer. At temperatures below +5°C, the application of acidic primers is not recommended, since the reaction may not proceed and moisture is retained in the layer.