Application of acid primer from a can begins with careful preparation of the surface, since moisture or grease gets under the layer phosphating coating will instantly start the oxidation process. Critical degrease the metal with a special compound and immediately after the solvent has dried, apply a thin layer Wash Primer, without waiting for re-oxidation of the cleaned area. It is a mistake to try to grind the rust down to bare metal and leave the part in the air - a chemical reaction with oxygen will begin in seconds, reducing the adhesion of the future paintwork.
The main task of this type of primer is not leveling, but creating a chemical bond between the steel or aluminum and subsequent layers of putty or acrylic. Phosphoric acid, contained in the composition, converts remaining oxides into a durable film, preventing the spread of corrosion under the paint. When working with aerosol cans, it is necessary to take into account the high pressure and drying speed of the solvent, which requires a certain skill from the master and maintaining a spraying distance.
If you ignore the technology and apply too thick a layer, the soil will not dry inside, and upon subsequent heating in the sun or in the drying chamber, swelling will begin. Proper application of acid primer from a can involves working in several passes with mandatory inter-layer drying, which we will discuss in detail below. Ignoring the exposure time leads to the fact that the acid base does not have time to react with the metal, and the protective effect is reduced to zero.
Principle of action and features of acid compositions
Acidic soils, often called phosphating or Wash Primer, work on the principle of metal passivation. Unlike epoxy sealants, which simply create a barrier, the acidic compound reacts with the surface to create a rough microstructure for better adhesion. The active component here is orthophosphoric acid, which dissolves microscopic oxides and converts them into insoluble phosphates.
The peculiarity of aerosol packaging is that the components are often already mixed or require minimal shaking, while two-component can systems require the addition of an activator. Reactive soil from a spray can is convenient for local repairs, where there is no point in diluting a whole liter of material. However, the lifespan of the finished mixture in the cylinder is limited, so after the first use the valve may dry out if the cylinder is not turned over and the nozzle is not blown out.
⚠️ Attention: Acid primer cannot be applied to surfaces previously treated with putty or acrylic primer. It is intended exclusively for pure metal (steel, aluminum, galvanized). Trying to apply it on top of old paintwork will result in peeling.
It is important to understand the difference between acid and epoxy primer. The first requires covering with an acrylic filler, since it itself is hygroscopic and is afraid of moisture. The second is the final insulator. If you have applied Wash Primer, you must cover it with a layer of acrylic filler primer before you start painting or puttying (although puttingtying with acid is also not recommended by manufacturers).
Necessary tools and preparation of the work area
For quality work, you will need not only a can of primer, but also a number of auxiliary tools. The quality of surface preparation determines 80% of the success of the entire process. Before you open the tank, make sure you have everything you need on hand to create the ideal conditions.
- 🧤 Personal protective equipment: a respirator with filters for organic vapors, safety glasses and nitrile gloves, as acid and solvent vapors are toxic.
- 🧼 Degreaser: special anti-silicone solvent that does not leave a greasy film after evaporation.
- 🌡️ Thermometer and hygrometer: to control surface temperature and indoor humidity, which are critical for chemical reactions.
- 🔧 Abrasives: sandpaper P80-P180 for initial sanding and P240 for finishing.
The work area should be well ventilated, but without drafts that could blow dust onto fresh soil. The air temperature should be between +15°C and +25°C. If the room is colder, the chemical reaction will slow down and the soil may not work. If it’s hotter, the solvent will evaporate too quickly, causing surface defects.
Use lint-free wipes for degreasing. Regular rags can leave lint that will show up under the paint.
Before starting work, be sure to shake the can vigorously for 2-3 minutes. There is a metal ball inside that helps mix the ingredients and break up any clumps. If this is not done, consistency the sprayed material will be uneven, which will lead to spotting and varying degrees of adhesion in different areas of the part.
Application technology: step-by-step instructions
The application process requires strict adherence to the sequence of actions. First, the damaged area is mechanically cleaned to bare metal. Then follows the degreasing stage: generously moisten a cloth with solvent and wipe the surface, then immediately wipe dry with a clean cloth. Do not rub too hard to avoid smearing the dirt.
☑️ Checklist before spraying
Keep the can at a distance of 20-25 cm from the surface. You need to apply the acidic primer from a spray can in a thin, “foggy” layer. Do not try to cover the metal with a thick layer the first time - this is the main mistake. The first layer should be translucent, its task is to wet the metal and start the reaction. Let it dry for 10-15 minutes at room temperature.
The second layer is applied similarly, also very thinly. The total coating thickness should not exceed 8-10 microns. If you apply too much material, it will remain sticky and will not dry, as the solvent will have nowhere to evaporate through the thick crust. Drying time between layers is critical for the evaporation of volatile fractions.
| Parameter | Meaning/Action | Note |
|---|---|---|
| Spray distance | 20-25 cm | Approaching will cause drips, moving away will cause overspray. |
| Drying time (interlayer) | 10-15 min | Depends on air temperature |
| Number of layers | 1-2 layers | Only thin "foggy" sprays |
| Complete polymerization | 30-60 min | Before applying acrylic primer |
After drying, the surface should become matte and slightly rough to the touch. If you see glossy spots, it means that the layer in this place is too thick or the surface has not been properly degreased. In this case, it is recommended to carefully sand the defect with a fine abrasive (P400-P600) and apply another thin layer of primer.
Drying time and intercoat exposure
Drying time for acid soil is a relative concept and depends on many factors. Formally Wash Primer dries "touch" in 10-15 minutes at +20°C. However, it takes longer for a complete chemical reaction and evaporation of solvents. If you rush and apply the acrylic primer too early, the acid solvents may become trapped under the new coat.
As the temperature increases, the drying time is reduced, but there is a risk of the solvent boiling in the pores of the metal. When the temperature drops, the process can take up to several hours. Forced drying infrared lamps are allowed, but only after preliminary atmospheric drying for 20 minutes, so that the bulk of the solvent has time to evaporate.
Effect of humidity on drying
At humidity above 80%, the drying time increases by 1.5-2 times. In addition, high humidity can lead to clouding of the soil (the “whiteness” effect) due to condensation of moisture in the fresh layer.
Readiness for further work can be determined visually and tactilely. The surface should be completely dry, matte and not sticky when touched lightly with a gloved finger. If traces of soil remain on the glove or if it feels sticky, drying must be continued. Overexposure (too long drying) is also undesirable: after 12-24 hours the adhesive properties of the surface may begin to decrease and light sanding will be required.
Compatibility with other materials and puttying
One of the most common questions: is it possible to putty directly over acidic soil? Technically, some manufacturers allow this, but professional painters recommend avoiding such proximity. Polyester putties contain styrene and peroxides, which may conflict with acid residues, causing swelling or poor adhesion.
The ideal scheme looks like this: metal -> acid primer (1-2 layers) -> acrylic primer-filler (2-3 layers) -> putty (if leveling is needed) -> acrylic primer again -> base. If the terrain allows, it is better to apply putty to bare metal or epoxy primer, and use acid for final protection of difficult areas before the main priming.
- 🚫 Cannot be applied: epoxy primers directly on acidic ones without an intermediate layer (poor adhesion).
- ✅ Can be applied: acrylic fillers, acrylic paints (through primer), liquid putties (with caution).
- ⚠️ Caution: straight enamels may react with acid, an insulating layer is required.
⚠️ Attention: Never apply putty over a thick layer of acidic primer. The putty should be applied either to the metal or to the epoxy primer. The acid layer is too thin and fragile to hold the volume of putty.
If you do decide to use putty over an acid primer, make sure that the primer is completely polymerized, and use only high-quality two-component putties with aluminum powder or fiberglass, which are less sensitive to chemical influences. However, the risk of future detachment remains high.
Typical errors and ways to resolve them
Violation of technology often leads to defects that appear several months after painting. The most common mistake is to apply a thick "wet" layer in hopes of covering the metal faster. This causes the top crust to dry out, leaving liquid solvent inside. When heated, it expands and bubbles up the paint.
Another mistake is working in a cold room. At temperatures below +10°C, phosphoric acid practically stops reacting with the metal. You are simply creating a decorative layer that does not perform a protective function. In such cases, it is necessary to use heat guns to heat the part itself, and not just the air around it.
The main rule: Acid soil is not a filler or an insulator. This is an adhesion primer. It should be barely visible on the metal.
Poor degreasing is the number one cause of peeling. If there are traces of silicone, oil or even finger sweat on the metal, the primer will fall on this film and not on the metal. Over time, this film will break down and a piece of paint will fall off along with the rust underneath. Always use two stages of degreasing: washing and final wiping.
Using an expired can can also cause problems. The components inside may separate or crystallize. If, when shaking, you do not hear a clear knock of the ball or feel that there is something hard inside, it is better not to risk it and buy a new cylinder. It will be more expensive to redo the repair.
Frequently asked questions (FAQ)
Is it possible to paint directly over acidic primer without acrylic filler?
No, you can't. Acidic soil is hygroscopic (absorbs moisture) and has no hiding power. The paint will lie unevenly, and moisture from the air will reach the metal, causing corrosion. Be sure to cover it with 2-3 layers of acrylic filler primer.
Do I need to sand the acid primer before applying acrylic?
Usually no, if no more than 24 hours have passed since application. Acidic primer creates excellent peel-off adhesion. If more than a day has passed, it is recommended to lightly treat the surface with Scotch Brite (P800-P1000) or fine sandpaper to refresh the surface and remove possible dust.
Is acid primer suitable for galvanized metal?
Yes, it is one of the best options for galvanizing, electroplating and aluminum. Ordinary soils do not adhere well to them, but acidic (phosphating) soils create the necessary chemical bond. However, special epoxy primers are also excellent for galvanizing.
How long does acid soil dry at +20 degrees?
Touch-drying time is 10-15 minutes. Complete readiness for applying acrylic primer is in 30-60 minutes. Complete polymerization and readiness for grinding (if required) - after 12-24 hours.
How to remove acidic soil from clothes or skin?
From the skin - only with plenty of warm water and soap until it dries. If it’s dry, mechanically, as the skin renews itself. Fresh soil is washed off from clothes or tools with acetone or solvent 646/647. Hardened soil from tools can be removed mechanically or by soaking in a strong solvent.