Direct application of acid primer to degreased metal is the only way to ensure future coating adhesion. If you skip the stage of primary chemical processing or violate the mixing proportions of the components Wurth or Motip, after a few months, pockets of corrosion will appear on the body under a new layer of paint. Phosphate primer reacts with iron oxides, creating a durable crystalline film that prevents rust from spreading further.
Working with this material requires strict adherence to safety precautions and technology, since the chemical reaction occurs instantly after mixing the base and activator. Incorrect application will result in the layer not drying or, conversely, becoming brittle and peeling off along with the putty. In this manual we will look at all the nuances of surface preparation, mixing components and quality control of the resulting coating.
The main mistake beginners make is trying to apply too thick a layer or using the material as a topcoat. Acid primer is a purely adhesive layer, the task of which is to “tie” subsequent materials to the bare metal. Two-component composition cannot be left open for a long time, as it quickly polymerizes and becomes unsuitable for work.
⚠️ Warning: Vapors from acid solvents are toxic. Carry out all work only in a well-ventilated area or outdoors, using a respirator with carbon filters and safety glasses.
Chemical composition and principle of action of reactive primer
The material is based on polyvinyl butyral resin dissolved in organic solvents and phosphoric acid, which acts as an activator. When the acid hits metal, it begins to displace oxygen from iron oxides, turning loose rust into a stable layer of zinc or iron phosphates. This process is called surface passivation, it stops corrosion and creates a rough texture for better adhesion.
Unlike epoxy primers, which create an insulating film over the metal, acid primers work at the molecular level. It penetrates into the microscopic pores of the metal, providing mechanical and chemical adhesion. Reoflex and other popular brands use various additives to speed up drying, but the basic principle remains the same for all products of this type.
It is important to understand that after drying the layer becomes thin and porous, so it does not have anti-corrosion resistance on its own. It must be covered with an acrylic filler primer or putty, which will create a barrier to moisture and oxygen. Without a top protective layer, acid can remain in the pores and cause a backlash over time.
- 🧪 Phosphoric acid - converts metal oxides into insoluble phosphates.
- 🧴 Polyvinyl butyral - creates an elastic base for adhesion to subsequent layers.
- 💨 Solvents — provide the necessary viscosity and evaporation rate.
Compatibility with other materials
Acid primer should not be applied directly under polyester putty, as the acid can disrupt its polymerization process. First, a thin layer of primer is applied, then it is covered with acrylic filler, and only after that can putty be used, if the manufacturer’s technology allows it.
Necessary tools and protective equipment
To perform the work efficiently, you will need a standard set of painting equipment. The main tool is a spray gun with a nozzle of 1.3–1.5 mm, although aerosol cans can be used for local repairs. When working with two-component compositions, the jar must have a measuring cup and a stirring stick.
Surface preparation requires a sander with P80-P120 abrasive to remove old paint and strip rust to bare metal. Degreasing is carried out with special anti-silicone compounds and lint-free wipes. The use of rags is unacceptable, as lint may remain on the surface and impair adhesion.
Pay special attention to personal protective equipment. Acidic soil contains aggressive components that can cause chemical burns to the skin or damage the respiratory tract. Respirator, safety glasses and nitrile gloves are a mandatory minimum for the master. Work clothes must cover all exposed areas of the body.
| Tool | Purpose | Requirements |
|---|---|---|
| Spray gun | Application of primer | Nozzle 1.3–1.5 mm, pressure 2–3 atm |
| Sander | Metal stripping | Abrasive P80–P120 |
| Degreaser | Oil removal | Anti-silicone, lint-free cloth |
| Measuring cup | Dosage of components | Graduation in proportions 1:1 |
Surface preparation before application
The quality of the final coating depends 90% on the preparation of the base. The metal must be absolutely clean, dry and matte. All remnants of old paint, rust and corrosion must be removed mechanically. If islands of rust remain on the surface, the acid will preserve them, but will not completely remove them, which can lead to swelling of the paint in the future.
After mechanical cleaning, the surface is thoroughly degreased. Movements with the napkin should be progressive, without circular rubbing, so as not to smear the dirt. After the first pass, change the napkin to a dry one and wipe the surface dry. Antisilicone must completely evaporate before priming.
Do not apply primer to wet or cold metal. The surface temperature should be above +10°C, the optimal range is 20°C. If a part has just been brought from outside in winter, it should be allowed to warm up in a warm room for at least 2–3 hours to avoid condensation.
⚠️ Attention: Do not use gasoline, kerosene or solvent 646 for degreasing. They leave a greasy film that will reduce the adhesion of the primer. Use only specialized automotive degreasers.
Technology of mixing and applying primer
Acid primer is sold in the form of two components: a base and an activator (hardener). Before use, they must be mixed in the strict proportions indicated on the package. The most common ratio is 1:1, but some manufacturers, such as Novol Protect, may require different proportions. Mixing is done immediately before application, since the pot life of the finished mixture is only 30–60 minutes.
After adding the activator, the mixture is thoroughly mixed for 2–3 minutes. It is important to stir the composition at the bottom and walls of the container so that there are no unmixed areas left. The finished primer is poured into the spray gun tank through a funnel filter, which will trap possible lumps and debris.
Application is carried out in 1–2 thin layers. The first layer (wet) is applied from a distance of 15–20 cm, covering the previous pass by 50%. Do not try to immediately cover the metal with a dense layer - this will cause drips and disrupt the chemical reaction. The second layer is applied 5-10 minutes after the first one becomes matte.
☑️ Checklist before painting
The pressure on the spray gun should be set in the range of 2–3 atmospheres. Too high a pressure will result in excess material consumption and the formation of a “fog” that will settle around, but will not lie on the metal. Too low pressure will not allow the formation of a correct spray pattern, and the soil will lie in stripes.
For hard-to-reach places and internal cavities of thresholds, it is convenient to use acidic primer in aerosol cans. It is ready for use and has a special tube for spraying at any angle.
Drying time and coating quality control
The drying time of acid soil depends on the ambient temperature and the thickness of the applied layer. At a temperature of +20°C and normal humidity, the surface dries “touch” in 10–15 minutes. Complete polymerization occurs within 30–40 minutes. After this, you can apply acrylic primer filler or putty.
Quality control is carried out visually and tactilely. A properly dried layer should be matte, uniform, without glitter or streaks. The color can vary from light yellow to greenish, depending on the brand and reaction with the metal. If the surface is sticky or has a glossy sheen, the layer is too thick or the mixing ratio is incorrect.
In some cases, especially when working with galvanized metal or aluminum, the color of the primer may not change as clearly as on black steel. This is normal, since the passivation reaction on non-ferrous metals occurs differently. The main thing is to make sure that the layer is dry and not smeared with a gloved finger.
| Parameter | Value at +20°C | Value at +10°C |
|---|---|---|
| Touch dry | 10–15 min | 20–30 min |
| Ready for sanding | 30–40 min | 60 min |
| Pot life of the mixture | 60 min | 90 min |
| Layer thickness | 5–10 µm | 5–10 µm |
Acid primer does not require sanding before applying acrylic filler if it has dried within the recommended time (up to 24 hours). If more than a day has passed, the surface must be matted with Scotch Brite.
Typical errors and ways to resolve them
One of the most common mistakes is applying primer to a poorly prepared surface. Remaining oil, moisture or oxides will cause the primer to peel off along with subsequent layers of paint. In this case, the right solution is to completely remove the coating down to the metal and repeat the preparation cycle.
Violation of mixing proportions also fatally affects the result. An excess of activator will make the layer brittle and cause corrosion, while a deficiency will prevent the reaction from proceeding to completion and the soil will remain sticky. Always use measuring cups and read the instructions on the jar carefully.
Trying to apply acid primer over putty or old varnish is unacceptable. It is intended for pure metal only. If applied to putty, the acid can destroy the structure of the polyester material, causing it to swell. There are other types of primers for such surfaces.
⚠️ Attention: Do not leave an open jar of diluted soil for later. The mixture quickly loses its properties and after an hour turns into an unusable mass. Dilute as much as you can use in 30–40 minutes.
FAQ: Frequently asked questions
Can acid primer be applied to aluminum?
Yes, it is possible and necessary. Aluminum actively oxidizes in air, forming a film that interferes with adhesion. Acidic soil binds well to the aluminum oxide film, providing reliable protection. The application technology is the same as for steel.
Do I need to sand the acid primer before applying acrylic?
If less than 24 hours have passed since application, sanding is not required. Acrylic primer can be applied directly. If more than a day has passed, the surface must be treated with Scotch Brite (P400–P600) to improve adhesion.
What is the difference between acid primer and epoxy primer?
Acid (phosphate) primer works on the principle of a chemical reaction with metal and creates a thin adhesive film. Epoxy primer creates a thick, moisture- and oxygen-impermeable insulating film. Often they are used together: first acid for adhesion, then epoxy for protection.
How long does it take for acidic soil to dry at low temperatures?
At temperatures below +10°C, the polymerization process slows down significantly. Drying time may increase 2–3 times. It is not recommended to carry out work at temperatures below +5°C, as the reaction may not complete.