Recovery of a car’s paintwork is a process that requires not only precision, but also a deep understanding of the chemical reactions that occur on the metal surface. One of the most important stages of body preparation is the primary processing, and there is often confusion between the different types of primers. Beginners in body repair often wonder: what exactly is needed for acidic Can it be replaced by more affordable alternatives? The answer to this question determines the durability of your repair and the metal’s protection from corrosion for years to come.
Acid soil, also known as phosphate primer, is a two-component composition based on polyvinyl butyl resins and orthophosphoric acid. Its main task is not to level the surface, as many people think, but to create a chemical bond between the bare metal and subsequent coating layers. Orthophosphoric acid reacts with iron oxides, converting rust into stable compounds and creating a protective film. This is the foundation on which the entire system of body protection from an aggressive external environment rests.
It is important to understand that improper use of this material can negate all further efforts. If you apply incompatible materials on top of it or break the drying technology, the coating can swell or peel off after a few months. In this article we will analyze in detail the chemical properties of the composition, the rules of its application and the typical mistakes that even experienced masters make when working with the chemical. phosphate.
Chemical composition and principle of operation
The basis of any high-quality acid soil is polyvinylbutyl resinswhich provide adhesion, and Orthophosphoric acid, responsible for the passivation of the metal. When applied to the surface, a complex chemical reaction occurs: the acid dissolves microscopic oxides and contaminants that cannot be removed mechanically, and converts them into insoluble phosphates. This process is called phosphation, and it creates a thin layer on the metal that prevents further corrosion spread.
Unlike acrylic or epoxy soils, the acidic composition does not create a thick insulating film. Its thickness after drying is only a few microns, making it ideal for processing hard-to-reach places, edges and welds. Active components They penetrate deep into the pores of the metal, providing protection even where mechanical cleaning was impossible. It is due to this property that acid soil is often called “liquid zinc”, although zinc may not be in its composition.
⚠️ Attention: Acid soil is a chemically active substance. Do not allow the composition to get on the exposed areas of the skin or in the eyes, as orthophosphoric acid can cause serious chemical burns. Only work in a well-ventilated room using a respirator.
The principle of the material is based on the creation of a barrier layer that blocks the access of oxygen and moisture to the metal. However, it is worth remembering that this layer is hygroscopic and can absorb moisture from the atmosphere if it is not covered with the finishing coating. Therefore acid-primer Always requires the application of secondary soil or paint in strict accordance with the time intervals specified by the manufacturer.
Can I use acid soil over old paint?
No, acid soil is only for bare metal. Applying it over paint or putty makes no sense, since the chemical reaction of phosphation will not occur, and the adhesion will be weak.
Why do you need acid soil in body repair
Primary appointment phosphate - protection of ferrous metal from corrosion in places where the factory integrity of the coating has been violated. It is ideal for welding after body repairs, door edges, sills and arches where the risk of rust formation is highest. It is also used to protect the internal cavities of the body, using special spray nozzles for hard-to-reach places.
This material is often used as a “transition bridge” between metal and epoxy soil, although experts are divided. Some manufacturers allow this combination under certain conditions, others categorically prohibit mixing products of different chemical groups. acidic layer It provides chemical bonding, and epoxy provides mechanical protection and insulation. However, it is safer for beginners to choose one material system to avoid compatibility issues.
Another important area of application is the processing of aluminum and galvanized surfaces. Aluminum is prone to rapid oxidation, forming an oxide film on the surface, which interferes with the adhesion of the paint. The acid soil effectively removes this film and creates a rough structure for reliable adhesion. Galvanized metalThe phosphate, which is often used in the modern automotive industry, also requires special training, which is provided by the phosphate primer.
- 🚗 Processing of welded seams and docking places of body elements after repair.
- 🛡️ Protection of door edges, sills and arches from chipping and subsequent corrosion.
- 🔧 Printing aluminum parts, radiators and discs before painting.
- 🏗️ Preparation of galvanized surfaces to ensure the adhesion of paint materials.
Acid soil is not designed to level surface defects, its task is chemical protection and adhesion, so do not try to fix scratches or dents with them.
Differences from epoxy and acrylic soil
Understanding the difference between soil types is key to successful repairs. epoxy creates a dense, impermeable to moisture and oxygen film, completely insulating the metal from the external environment. It has excellent anti-corrosion properties, but its adhesion to the metal is more mechanical in nature, requiring good surface roughness. Unlike him, acidic It works at a chemical level, penetrating the structure of the metal, but does not provide such a powerful physical protection from shocks.
Acrylic soils, in turn, are designed primarily for leveling the surface and filling the risks from abrasive. They have a large thickness of dry residue and are easily grinded. Acid primer You can not grind (or you can only slightly if a long time has passed), since you risk sanding the entire protective layer. Acrylic is applied on top of epoxy soil or directly to metal (if technology permits), but never replaces the primary chemical protection function.
| Characteristics | Acid soil | epoxy | Acrylic soil |
|---|---|---|---|
| Type of protection | Chemical (phosphate) | Physical (barrier) | Aligning |
| Thickness of layer | Minimum (8-13 μm) | Medium (up to 100 μm) | High (up to 300 μm) |
| Grinding | Not recommended | Required before painting | Required. |
| Compatibility | Requires overlapping. | Universal framework | Finishing paint |
The choice between these materials depends on the condition of the body and the planned budget of work. If the metal is pure, without corrosion, and you want to make "for centuries", often recommend a bundle: acidic for adhesion + epoxy Isolation. If the budget is limited or repair cosmetic, you can limit yourself to high-quality epoxy soil, which also shows excellent results on the prepared surface.
Application technology and drying time
Application process acid-soil It requires strict adherence to the proportions of mixing components. It’s usually 1:1, but always check the instructions on the manufacturer’s bank. The mixed material retains its properties for a short time (viability), usually from 30 minutes to 2 hours, after which it begins to thicken and loses its adhesive properties. Therefore, the mixture should be prepared immediately before work.
The composition can be applied with a spray gun (HVLP or LVLP) with a duse of 1.3-1.5 mm or with an aerosol can in hard-to-reach places. It's important not to overdo it. acid-primer It is applied in one thin "wet" layer. Excessive thickness can lead to the fact that the upper part of the layer will dry, and the lower part will remain sticky, which will negatively affect the adhesion of the next coating. The pressure at the exit of the gun should be within 2-3 atmospheres.
☑️ Preparation for acid soil application
The drying time depends on the ambient temperature and humidity. At +20°C, surface drying takes 10-15 minutes, complete polymerization takes about 2 hours. However, for the application of secondary soil or paint, it is often enough to withstand a technological pause of 30-40 minutes. Temperature regime critically important: at low temperatures, the phosphate reaction may not go through to the end, and at too high - the solvent will evaporate faster than the film will have time to form.
⚠️ Attention: Do not attempt to accelerate acid soil drying with infrared lamps or a building hair dryer. Intense heating can lead to boiling of the solvent and the formation of pores, which will destroy the protective layer.
Compatibility with other materials
The issue of compatibility is one of the most acute in body repair. Acid soil It is not compatible with polyester putty. If you apply a putty on top of an acidic or vice versa, after some time, the material will detach due to a chemical conflict of components. Spattle can only be bare metal or epoxy soil, but not phosphate compounds.
Compatibility with epoxy soils is a matter of debate. Some manufacturers (e.g., manufacturers) Novitol or Body) indicate the possibility of applying the epoxy soil over the acidic soil after complete polymerization and easy grinding. Other brands recommend avoiding such a “pie”, arguing that epoxide itself has excellent adhesion, and the acid layer can become a weak link. If you use products of the same system, follow the manufacturer’s recommendations; if you mix brands, it is better to be safe.
Acrylic soils and paintwork are compatible with phosphate without limitation, provided that the required drying time has passed. The paint is safely placed on the prepared surface, and the risk of defects is minimal. The main rule is never to leave the acid soil open for a long time (more than 24 hours) without overlapping, as it can begin to absorb moisture from the air.
Frequent mistakes when working with acid soil
One of the most common mistakes is the application of acid-soil on already painted surfaces or on the putty. As mentioned, the chemical reaction only occurs with metal. Trying to “strengthen” the adhesion on the old layer of paint, the master only creates an intermediate layer, which over time can peel off along with the new paint. Always apply the primer only on the metal peeled to shine.
The second mistake is the violation of the mixing proportions. Adding more hardener "on the eye" will not accelerate drying, but will make the coating brittle. The lack of hardener will lead to the fact that the soil will remain sticky and will never dry. Use measuring cups with marking and strictly follow the instructions manufacturer.
The third mistake is ignoring the grinding of the metal before applying. Acid soil requires mechanical roughness (risks) for better adhesion. Smooth, polished metal is not the best base. Be sure to walk in an abrasive P180-P240 before applying the primer to create a microrelief.
- ❌ Applying putty on top of acid soil (will lead to detachment).
- ❌ Use on plastic parts (no reaction, only excess chemistry).
- ❌ Attempt to grind a wet or dry layer (abrasive will be clogged).
- ❌ Storage of an open can with mixed soil "for later" (the material will deteriorate).
⚠️ Attention: Do not use acid soil to protect the bottom of the car. For these purposes, there are special bitumen or polymer mastics that better withstand mechanical impacts of stones and gravel.
FAQ: Frequently Asked Questions
Can I paint immediately on acid soil without applying a secondary layer?
Technically possible, but not recommended for exterior body parts. Acid soil does not have sufficient cover and resistance to UV radiation. Over time, the paint may fade or lose its shine. Always use acrylic filler soil or base before the varnish.
Do I need to grind the acid soil before applying epoxy?
If less than 24 hours have passed since application and the surface is matte, grinding is usually not required. If more time has passed or there is gloss / contamination on the surface, light matting with abrasive is necessary. P320-P400 To improve adhesion.
What is the shelf life of mixed acid soil?
The viability of the mixture in a closed container is usually 6 to 12 months, but it is best to use it for 6 months. However, after mixing with the hardener, the material is suitable for use only for 1-2 hours (depending on temperature and manufacturer).
Is the acid soil suitable for galvanizing?
Yes, it is one of the best options for preparing galvanized surfaces. It provides the necessary adhesion, which is difficult to obtain with conventional soils. However, before applying galvanizing, it is still recommended to treat with abrasive (scuffing) to create roughness.
How to Dilute the thickened acid soil?
Use only special solvents recommended by the manufacturer (usually this is the case) for dilution. 647, 650 or branded diluents). Do not use acetone or gasoline, as these can disrupt the chemical formula and degrade the properties of the coating.