Question about the need to use primer (primers) occurs for everyone who has at least once encountered local body repairs or complete repainting of a car. Many beginners, trying to save time and budget, often consider skipping this step, believing that high-quality paint will lie smoothly on the stripped metal. However, such savings most often lead to sad consequences that have to be corrected at a much higher cost.
A primer is not just an additional layer, but fundamental basis the entire paint and varnish coating (LPC). It is the quality and type of primer applied that determines how long the paint will stay on the surface, whether corrosion will appear underneath it after six months, and how deep it will appear. metallic or mother of pearl after final drying. Ignoring this stage is tantamount to building a house without a foundation.
In this article, we will take a closer look at why professionals never skip the priming step, what chemical processes occur between layers, and whether exceptions to the rules can be found. You will understand the difference between adhesive and sealing compounds, and how to properly prepare the surface to achieve factory quality.
Primer functions: why the body needs it
The main task of any primer is to create a reliable link between materials that are completely different in nature. The metal of the body and the enamel with which we cover the car have different chemical structures and physical densities. Without an intermediate layer, the paint may simply roll off the surface or peel off in large pieces at the first mechanical impact or temperature change.
In addition to adhesion, soil performs a protective function. Metal, even after thorough cleaning, remains susceptible to moisture and oxygen. Special anti-corrosion additives in the primer block oxidative processes, preventing the appearance of rust under the paint layer. This is especially critical for sills, arches and the bottom, where the risk of contact with water and reagents is maximum.
⚠️ Attention: Applying paint to bare metal without a primer in 99% of cases will lead to peeling of the coating (the “egg shell” effect) and the rapid development of hidden corrosion in the first year of operation.
Another important function is microrelief leveling. Even after sanding, abrasive scratches remain on the surface. Liquid primer fills these microdefects, creating a perfectly smooth surface on which the paint will look like cast. The primer also prevents expensive enamel from being absorbed into porous surfaces such as putty or old matte varnish.
Use a primer in a contrasting color (such as gray or yellow) under light paints to better see the coverage of the base and avoid stains.
Types of automotive primers and their purposes
The automotive chemicals market offers many types of primers, and the choice of a specific product depends on the condition of the body and the type of finish coat. The wrong choice of soil can ruin all your efforts. For example, using an acidic primer under polyester putty will cause a chemical reaction that will destroy the adhesion.
For ease of selection, the main types of primers are summarized in the table below:
| Soil type | Base | Main purpose | Application |
|---|---|---|---|
| Acidic (Phosphate) | Phosphoric acid | Rust passivation, primary protection | Only under epoxy primer or putty |
| Epoxy | Epoxy resins | Sealing, moisture protection, insulation | On bare metal, over acidifier |
| Acrylic | Acrylic polymers | Leveling, filling scratches, base for paint | For epoxy, putty, old varnish |
| Adhesive (Velcro) | Special polymers | Adhesion to smooth surfaces (plastic, glass) | Apply a thin layer to difficult surfaces |
The most universal and often used in body repair is acrylic primer. It has excellent filling power, is easy to sand and is compatible with most enamels. However, for working with “bare” metal after removing rust, the combination that is best suited is: acid etching agent + epoxy insulator.
Separately, it is worth mentioning primers for plastic. Conventional compounds do not adhere well to such parts due to the elasticity of the material. This requires a special a plastic primer that penetrates the polymer structure and creates a rough, sticky surface for subsequent application of acrylic.
Is it possible to mix different types of soil?
It is strictly not recommended to mix primers on different bases (for example, epoxy and acrylic) in one jar. This will lead to curdling of the composition and loss of properties. Layers of different primers can be applied, but only using the “wet on wet” technology or after complete drying and matting.
Application technology: step-by-step instructions
The priming process requires strict adherence to technology. Errors at this stage, such as temperature violations or incorrect layer thickness, can be fatal to the quality of the repair. First you need to prepare the surface: it must be clean, dry and grease-free.
If you're working with bare metal, the first step is often to apply a thin mist of acid primer. After it dries (usually 15-20 minutes), epoxy primer is applied to create an airtight film. Only after the epoxy has completely dried (often requiring several hours or a day) can you move on to acrylic filler if leveling is required.
☑️ Preparation for priming
Acrylic primer is applied in 2-3 layers with interlayer drying (flash-off) for about 10-15 minutes. It is important not to soak the surface immediately to avoid the putty rising or bubbles forming. Each layer should be thin and even. After complete drying (usually 24 hours at 20°C), the primer is sanded with abrasive P400-P600 under the base.
To achieve an ideal result, it is necessary to maintain the distance to the surface and the angle of inclination of the spray gun. Movements should be smooth, overlapping the previous pass by 50%. Violating these rules will result in streaks (bullseyes) that will appear after painting.
Is it possible to paint without a primer: myths and reality
On the Internet you can find opinions that modern paints (especially water-based or some types of “rust enamels”) allow you to ignore the primer. This is a dangerous misconception. Yes, there are “3 in 1” paints containing a rust converter and primer, but their use is only justified for temporary repairs or elements hidden from view (for example, the inside of the spar).
For external body parts that are exposed to ultraviolet radiation, moisture, stones and temperature changes, the absence of a full primer layer is unacceptable. Paint applied directly to metal or putty does not have sufficient adhesion. Over time, moisture gets into the micropores, the metal begins to oxidize, and the paint swells.
The only exception is when you can do without applying acrylic filler - this is painting an absolutely intact factory part, from which the gloss has only been removed (matted) for repainting in a different color. In this case, the old factory primer and base serve as the base. But if there is even the slightest doubt about the integrity of the old paintwork or stripping down to metal was carried out, primer is required.
⚠️ Attention: Never apply polyester putty to acidic primer. The acid contained in the etching agent will not have time to evaporate and will disrupt the polymerization process of the putty, which will lead to its peeling.
Common mistakes when working with soil
Even experienced craftsmen sometimes make mistakes that can cost the client the quality of the coating. One of the most common problems is insufficient drying. If you apply paint to under-dried soil, enamel solvents can preserve moisture or solvent residues inside the layer. This will cause dull spots, changes in shade (especially on metallics) or even paint bubbling.
Another common mistake is using incompatible materials. For example, applying an aggressive acrylic enamel solvent to a weak, low-quality primer can cause it to wrinkle. Always use products from the same line or check material compatibility tables from different manufacturers.
The quality of surface preparation before priming is also critically important. The presence of dust, oils or moisture on the metal will cause the soil to form defects (craters, fish eyes). These defects will then have to be sanded out for a long time and tediously, at the risk of rubbing the layer down to the metal.
The quality of the final coating depends 80% on the quality of surface preparation and the correct choice of primer, and not on the cost of the paint itself.
Drying time and preparation for painting
The drying time of the primer varies depending on the ambient temperature, the thickness of the applied layer and the type of product. Standard acrylic primer dries in about 24 hours at +20°C. However, in professional dryers at temperatures +60°C this process takes only 30-40 minutes.
It is important to distinguish between the touch-drying time (when the dust does not stick) and the time of complete polymerization. The primer can only be sanded after it has completely cured. If you start sanding too early, the abrasive will become clogged, and the soil itself may drag behind the sandpaper, leaving deep grooves.
Before applying the base, the primed surface must be thoroughly blown with compressed air, especially in recesses and hidden cavities, and degreased. Using a tack cloth (anti-static) before painting will help remove any tiny lint or dust that may have settled on the surface after sanding.
Do I need to sand the epoxy primer before applying acrylic?
Yes, if more than a day has passed since the application of the epoxy primer (maximum curing period), it must be matted with abrasive P320-P400 to ensure mechanical adhesion of the next layer. If the primer is applied "wet on wet" within the processing window (usually 24-48 hours), sanding is not required.
Is it possible to dilute the soil with water?
No, most automotive primers (epoxy, acrylic) are solvent-soluble. Diluting them with water will lead to separation of the composition and loss of properties. Water-borne soils exist, but they require special equipment and drying conditions that differ from standard ones.
How many layers of primer is optimal to apply?
Typically, 2-3 wet layers of acrylic primer are applied with drying between layers. The first layer is often made thin (nailed) to ensure adhesion, and subsequent layers are more sprayed to fill the scratches. Too thick a layer can lead to drips and long drying times.
What to do if the soil is shagreen?
If the shagreen (orange peel) on the ground is large, it must be sanded off with an abrasive P400-P500 until a smooth surface is obtained. If the shagreen is small and you plan to apply a thick layer of base or varnish, a light matting is sometimes allowed, but for an ideal result the primer should be smooth.
What abrasive should I use for sanding the primer?
Under acrylic and water-based base enamels, the primer is usually sanded with a gradation abrasive P400 or P500. Using a coarser abrasive (P240-P320) may leave marks that will appear after painting, especially on dark colors and metallics.