Acid primer for a car: instructions for use with photos and advice from professionals

Acid primer (also known as reactive, phosphating or etching) is a specialized compound that is used in body repair to prepare metal before painting. Its main task is to create a strong adhesion between bare metal and subsequent layers of primer/paint, as well as protect the surface from corrosion. Without the correct application of acid primer, even the most expensive paint will begin to peel off after a few months, and rust will show through the fresh paint layer.

Many car owners and novice painters make critical mistakes when working with this material: applying it to a dirty surface, not observing the exposure time, or using incompatible components. The result is bubbles in the paint, poor adhesion and repeated repairs after six months. In this article we will analyze step-by-step technology for using acid soil, from metal preparation to finishing, taking into account the nuances for different types of damage (chips, rust, bare metal after welding).

We will pay special attention brands (for example, Novol Protect 340, Body 960, Reoflex Acid Primer) and their features, and also debunk the myths about the “versatility” of acid soils. If you are repairing a car with your own hands or supervising the work of painters in a service center, these instructions will help you avoid punctures and save on rework.

1. What is acid soil and why is it needed?

Acid soil is a two-component composition based on phosphoric acid, which enters into a chemical reaction with the metal, forming a protective film on its surface. Unlike conventional leveling primers (for example, epoxy or acrylic), it does not just “stick” to the surface, but literally etches the top layer of metal at the micro level, creating a porous structure for better adhesion.

Main functions of acid soil:

• 🛡️ Anti-corrosion protection — blocks pockets of rust and prevents its spread under the paint.
• 🔗 Improved adhesion — without it, paint or putty can peel off even with minor vibrations of the body.
• 🧪 Neutralization of rust residues - converts iron oxides into stable phosphates.
• 🔍 Defect detection — after application, microcracks and pores in the metal, invisible to the eye, appear.

Where it is used:

• 🚗 Bare metal after removing rust or welding.
• 🔧 Chips and scratches to metal (even small ones).
• 🔩 Places after straightening, where the factory protective layer is damaged.
• 🔄 Areas with residual rust, which cannot be removed mechanically (for example, deep in the pores).

⚠️ Attention: Acidic soil does not replace mechanical rust removal! It only fixes its remnants and prevents further spread. If there are loose layers of corrosion on the metal, they must be removed by grinding or sandblasting.

2. Types of acidic soils: which one to choose for your renovation

Not all acidic soils are created equal. Their composition and properties depend on the purpose and type of surface being treated. Here are the key differences:

Soil type	Composition	Scope of application	Drying time	Color after application
One-component	Ready solution of orthophosphoric acid	Minor repairs, chips, scratches	10–15 minutes	Transparent or gray
Two-component	Basic composition + activator	Large areas, weld seams, severe corrosion	20–30 minutes	Yellow, green or red
With zinc	Acid + zinc powder	Areas with a high risk of corrosion (thresholds, arches)	30–40 minutes	Gray (metallic shade)
For aluminum	Special additives for non-ferrous metals	Aluminum parts (hoods, fenders)	15–20 minutes	Transparent or yellow

Popular brands and their features:

• 🔹 Novol Protect 340 — universal two-component primer, suitable for steel and aluminum. Drying time: 20 minutes at +20°C.
• 🔹 Body 960 — contains zinc, ideal for sills and wheel arches. Requires mandatory coating with acrylic primer.
• 🔹 Reoflex Acid Primer — quick-drying (10–15 minutes), but sensitive to air humidity.
• 🔹 APP Fill&Prime - combined primer (acid + filler), simplifies the process, but is less reliable for severe corrosion.

How to choose:

• For minor repairs (chips, scratches) - one-component primer in a can (for example, Kudo KU-9001).
• For large plots (after welding, straightening) - two-component with activator.
• For aluminum - specialized compounds (for example, PPG DP40LF).
• For high anti-corrosion protection - soils with zinc (Body 960, Spies Hecker Permacron).

3. Surface preparation: the key to success

Acid primer is applied only for pure metal. Any remaining rust, oil or old paint will reduce its effectiveness to zero. Let's look at the preparation process step by step:

Remove rust mechanically (sandpaper P80–P120 or sandblast)|Degrease the surface (white spirit, antisilicone)|Dry (compressor or napkins)|Matt the surface (Scotch-Brite P240–P320)|Isolate adjacent areas (masking tape, paper)-->

1. Removing rust and old paint

• 🔨 For small areas: sandpaper P80–P120 (manually or using a sander).
• 💨 For large areas: sandblasting (optimal option).
• 🧲 For hard-to-reach places: chemical rust converters (for example, Tsinkar), but their remains must be washed off thoroughly!

2. Degreasing

Use antisilicone or white spirit. Never use solvent 646 — he leaves the film! Technology:

1. Apply the composition to a lint-free cloth.
2. Wipe the surface two movements (the first is removing dirt, the second is final cleaning).
3. Do not touch processed metal with your hands!

3. Matting

Acidic soil adheres better to rough surfaces. Use Scotch Brite P240–P320 or a gray abrasive sponge. The main thing is not to overdo it: a light matte is enough to remove the shine.

⚠️ Attention: If there are traces of welding (scale) left on the metal, they must be removed up to applying primer! Scale does not react with acid and will become a source of corrosion.

4. Step-by-step instructions for applying acid primer

Now we move on to the most important stage. Both the application technique and adherence to the exposure time are important here. Errors at this step lead to paint peeling or bubbling in 1–2 months.

Step 1. Preparation of the composition

• 🧪 For two-component soils: mix the base with the activator in the proportion indicated on the can (usually 4:1 or 3:1).
• ⏳ Pot life of the mixture — 2–4 hours (depending on the brand). After this, the soil begins to polymerize in the jar!
• 🌡️ The room temperature should be 18–25°C. At +10°C the drying time is doubled.

Step 2. Application

Use spray gun with nozzle 1.3–1.5 mm and pressure 2–2.5 atm. Technique:

• 🎨 Keep your gun at a distance 15–20 cm from the surface.
• 🔄 Apply 2–3 thin layers with intermediate drying for 3–5 minutes.
• 🚫 Don’t allow any leaks! Acidic soil cannot be sanded — mistakes will have to be washed off and reapplied.

Step 3. Drying

The time depends on the type of soil and conditions:

• 🕒 One-component: 10–15 minutes at +20°C.
• ⏰ Two-component: 20–30 minutes (until matte).
• 🔥 Forced drying (IR lamp, heat gun) reduces the time by 30–40%, but the temperature should not exceed 60°C.

Step 4. Quality check

After drying, the soil should:

• ☑️ To be evenly matte (shine means under-drying).
• ☑️ Do not stick to your finger (check with the back of your hand).
• ☑️ Do not have drips or “shagreen” (irregularities).

What to do if the soil has dried out with defects?

If streaks appear, wash them off carefully. paint remover (for example, Body 700), dry and apply a new coat. If the soil remains sticky after an hour, it means that the mixing proportion is incorrect or the room temperature is low. In this case, you need to remove the layer completely and repeat the process.

5. Common mistakes and how to avoid them

Even experienced painters sometimes make punctures when working with acidic soil. Here TOP-5 errorsthat lead to marriage:

1. Application on dirty metal

   Consequences: the primer does not react, the paint peels off in layers.

   How to avoid: degrease the surface twice - before and after matting.

2. Using expired soil

   Consequences: loss of adhesion, appearance of craters on the paint.

   How to avoid: check the production date on the can. Shelf life: 12–18 months.

3. Applying a thick layer

   Consequences: the soil does not dry out inside, später führt zu Blasenbildung (bubbles under the paint).

   How to avoid: apply thin layers with intermediate drying.

4. No intermediate sanding

   Consequences: poor adhesion of acrylic primer or paint.

   How to avoid: After acidic soil dries necessarily Apply a layer of acrylic leveling primer (for example, Novol Protect 360).

5. Working in high humidity

   Consequences: the appearance of a white coating (salt) on the ground.

   How to avoid: the humidity in the room should be <70%. Use a dehumidifier.

⚠️ Attention: Never apply acid primer to putty or old paint! It's intended only for bare metal. For other surfaces, use acrylic primer.

6. Compatibility with other materials

Acid soil cannot be used “alone”. It always comes in conjunction with other materials. Let's figure out what goes with what:

Material	Acid primer compatibility	Waiting time before application	Notes
Acrylic primer	✅ Yes	15–30 minutes	Required for surface leveling
Epoxy primer	❌ No	—	Blocks acid soil reaction
Putty	❌ No	—	Acid primer is applied up to puttying
Paint (base/varnish)	⚠️ Conditional	1–2 hours	Only after acrylic primer!
Primer for plastic	❌ No	—	Use special primers for plastic

Scheme of the correct layer pie:

1. Bare metal → acid soil → drying 15–30 min.
2. Acrylic primer leveler → drying 1–2 hours → sanding P320–P400.
3. Base paint → drying 10–15 min.
4. Varnish → drying 24 hours.

Exception: for minor repairs (chips, scratches), you can apply paint directly to the acidic primer, but only if:

• 🎯 Damage area < 5 cm².
• 🖌️ Used brush or airbrush (not a spray gun).
• 🌡️ Room temperature > +20°C.

7. Safety measures during operation

Acidic soil contains aggressive chemical components that can be harmful to health. Observe the following rules:

Respiratory and skin protection:

• 😷 Use respirator with filter type A2P2 (protection against organic vapors and acids).
• 🧤 In your arms - nitrile gloves (latex ones dissolve!).
• 👓 Glasses with side protection (soil splashes can damage the cornea).

Fire safety:

• 🔥 Acid soil highly flammable. Work away from open flames and sparks.
• 🚫 Smoking is prohibited within a radius of 5 meters from the work area.
• 💨 Provide supply and exhaust ventilation (or work outside in windless weather).

Waste disposal:

• 🗑️ Return the remaining soil and used napkins as chemical waste (do not throw it in household waste!).
• 💦 Rinse the gun only with a special solvent (for example, Sata Jet Clean).

⚠️ Attention: If soil gets on your skin, wash it immediately with plenty of soap and water. If soil gets into your eyes, rinse 15 minutes and consult a doctor!

8. Frequently asked questions about acid soil

Can acid primer be applied to aluminum?

Yes, but only specialized compounds for non-ferrous metals (for example, PPG DP40LF or Spies Hecker Permacron 5460). Regular acid primer for steel can corrode aluminum!

How many layers of acid primer should I apply?

Enough 1–2 thin layers. The main thing is uniform coverage without drips. The third layer is only needed when treating porous surfaces (for example, after sandblasting).

How to wash off acidic primer if it gets on paint?

Use paint remover (for example, Body 700 or Abro PR-600). Apply for 5-10 minutes, then rinse with water. Do not rub with a dry cloth - you can damage the paintwork!

Is it possible to paint directly over acidic primer without acrylic?

Technically possible, but highly not recommended. Acrylic primer leveler:

• 🔹 Fills micropores.
• 🔹 Improves paint adhesion.
• 🔹 Prevents the appearance of yellow spots (typical of some acidic soils).

The exception is minor repairs (chips < 1 cm).

How to store acid soil?

Storage conditions:

• 🌡️ Temperature: +5…+25°C.
• 💡 Away from direct sunlight.
• 🔒 Tightly closed container (acid reacts with air).
• 🕒 Shelf life: 12–18 months from the production date.

If the soil is cloudy or flaky, it cannot be used!