Cracks in the primer on a car body are a problem faced by both beginners and experienced craftsmen. At first glance, it seems that the defect is purely aesthetic, but in fact it signals serious violations in the preparation or painting technology. Soil acts as a link between the metal and the paintwork, and its cracking leads to corrosion, peeling of paint and even through damage to the body.

Many car owners mistakenly believe that cracks appear only because of “bad paint” or “cheap primer.” In practice, the reasons are much deeper: from improper drying to chemical incompatibility of materials. In this article we will look at 7 Key Factors, why the soil on a car cracks, and we will also give step-by-step instructions for eliminating the defect - from local repairs to complete refurbishment of the coating.

We will pay special attention hidden reasons, which are often overlooked: for example, the influence of residual stresses in the metal after straightening or errors when mixing soil components. You will also learn how to distinguish technological microcracks (acceptable) from critical defects requiring immediate intervention.

📊 Have you encountered cracks in the ground on your car?
Yes, after painting at the service center
Yes, after self-repair
No, but I heard about such a problem
I don't know what it looks like

1. Incorrect surface preparation before priming

Before 70% of cases Soil cracking is associated with errors at the metal preparation stage. Even microscopic particles of dirt, grease or rust disrupt adhesion, creating stress points. This is especially critical for aluminum bodies and galvanized parts where special processing is required.

Typical mistakes:

  • 🧴 Using a low-quality degreaser (for example, White spirit instead of specialized compounds like APP Wash Primer).
  • 🧂 Incomplete removal of abrasive after grinding - particles P80-P120 remain in the pores of the metal.
  • 🔥 Skip the phosphating or passivation stage on the “bare” metal.
  • 💦 Moisture entering the surface due to non-compliance with climatic conditions (humidity higher 60%).

On Audi and BMW with aluminum panels, the soil often cracks due to neglect acid etching (self-etch primer). Without it, the oxide film on the aluminum prevents adhesion, and after 6–12 months “cobwebs” appear.

⚠️ Attention: If you sanded the metal “dry” without a vacuum cleaner, abrasive particles are guaranteed to remain in the pores. They create microscopic “cushions”, which later lead to soil detachment.
How to check the quality of degreasing?

Apply a thin layer of distilled water to the metal. If water collects in drops, the surface is oily. If it spreads evenly, degreasing has been done correctly.

2. Incompatibility of materials: primer vs paint vs metal

Chemical incompatibility is the second most common cause of cracks. Manufacturers of paints and varnishes (PPG, Sikkens, Mobihel) clearly indicate which primers are compatible with their paints. For example, epoxy primer cannot be covered acrylic enamels without an intermediate layer of insulator.

Common conflicts:

Soil Incompatible paint Consequences
Epoxy (2K) Nitro paints Instant wrinkling
Acidic (Wash Primer) Polyurethane enamels Cracks after 3–6 months
Acrylic (1K) Metallics with high aluminum content Loss of shine + microcracks
Polyester ( for plastic) Alkyd enamels Delamination of layers

Critical mistake: using one brand of primer and another brand of paint without a compatibility test. For example, soil PPG DP40/DP40LF incompatible with paints Sikkens Autowave without insulating layer.

How to avoid the problem:

  • 📖 Always check technical cards manufacturer (available on websites PPG, Axalta).
  • 🧪 Make a test coating on a separate sheet of metal.
  • 🔄 When changing the brand of materials, use transitional soils (for example, PPG K36).

3. Violation of soil drying technology

The soil must dry under strictly defined conditions. Exceeding the temperature or insufficient exposure leads to internal stresses in the layer, which later appear as cracks. For example, 2K soils require drying 60°C for 30–40 minutes, and not “in the sun.”

Typical drying mistakes:

  • ☀️ Drying in direct sunlight (uneven heating → tension).
  • 🔥 Overheating in the drying chamber (over 80°C for epoxy primers).
  • ⏳ Insufficient inter-layer drying (less than 10 minutes between layers).
  • 💨 Drafts indoors (lead to uneven drying).

For water-borne soils (for example, PPG Aquabase) air humidity is critical. When the humidity is higher 70% water from the ground evaporates too slowly, which leads to microporosity and subsequent cracking.

⚠️ Attention: If you dry the soil with infrared lamps, keep them at a distance of at least 50 cm from the surface. Otherwise, the top layer will be “baked”, and the bottom will remain damp - these are guaranteed cracks in 1–2 years.

Chamber temperature: 60–70°C|

Humidity: no higher than 60%|

Interlayer drying: 10-15 minutes|

Cooling before painting: 30 minutes at 20°C-->

4. Too thick or thin soil layer

Optimal soil thickness - 80–120 microns. Exceeding this value leads to cracking due to internal stresses, and insufficient thickness leads to metal translucency and corrosion.

How to identify the problem:

  • 📏 Use thickness gauge (for example, Elcometer 456). Readings above 150 microns for soil - a critical signal.
  • 👁️ Visually: thick primer has a “plastic” shine, thin primer has a matte appearance and gaps.
  • 🧲 Magnetic test: if the magnet sticks too much, the layer is too thin.

On Japanese cars (for example, Toyota, Nissan) a common problem with local thickening of the soil in weld areas. Here the metal was heated during production, and the soil lays unevenly. The solution is additional sanding of these areas P180 before priming.

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To control layer thickness, apply primer to a test plate next to the part. After drying, measure the thickness and adjust the pressure of the spray gun.

5. Effect of residual stresses in metal

After straightening or welding Internal stresses remain in the metal, which later “push” the soil outward, forming cracks. This problem is especially relevant for:

  • 🚗 Car after an accident (even if the body was “pulled out” on the slipway).
  • 🔧 Parts with spot welding (thresholds, racks).
  • 🔥 Areas subject to local heating (for example, when cutting with a grinder).

How to minimize risks:

  1. After straightening anneal the metal cutting torch (temperature 600–700°C, air cooling).
  2. Use elastic soils (for example, PPG DP90 with additive Flex Additive).
  3. Apply primer to 2–3 thin layers with interlayer drying 15–20 minutes.

On German cars (Mercedes, Volkswagen) the soil on the thresholds often cracks due to residual stresses after factory welding. The solution is local grinding P240 + application epoxy primer with plasticizer.

6. Impact of external factors: temperature, chemistry, mechanics

Even perfectly applied primer can crack under the influence of external conditions. Let's look at the key risks:

Temperature changes:

  • ❄️ Frost below -20°C makes the soil brittle (especially acrylic).
  • 🌡️ Overheating under the hood (over 120°C) leads to bubbling.

Chemical exposure:

  • 🧪 Detergents with alkali (pH > 10) destroy acrylic primers.
  • 🛢️ Fuel or oil getting on dry soil (for example, when painting thresholds).

Mechanical loads:

  • 🪨 Flying stones from wheels (especially on bumpers and fenders).
  • 🚗 Vibrations when driving off-road (critical for the soil at the bottom of the car).

On pickup trucks and SUVs (Ford Raptor, Toyota Hilux) the soil often cracks on the bottom of the doors due to the combination sandblast wear + vibrations. The solution is to use shockproof soils (for example, 3M™ Scotchkote™).

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Epoxy primers are more resistant to chemicals, but are less resistant to mechanical loads. To protect the bottom, choose polyurethane or rubber primers.

7. Mistakes when mixing and applying primer

Incorrect proportions of components or application technique are a direct path to cracks. Let's look at the critical points:

Mixing:

  • 🧪 Failure to comply with the proportions of primer and hardener (for example, 4:1 instead of 2:1).
  • 🕒 Exceeding the pot life time of the mixture (at 2K soils — 30–60 minutes).
  • 🌡️ Use of cold components (optimum temperature - 20–25°C).

Application:

  • 🎨 The pressure in the spray gun is too high (optimally - 2–2.5 bar).
  • 🌀 Incorrect distance to surface (should be 15–20 cm).
  • 🔄 Applying “crosswise” without overlapping layers on 50%.

On Chinese cars (Geely, Chery) the soil often cracks due to use cheap spray guns with an uneven torch. The solution is to rent professional equipment (SATAjet, Iwata) or purchasing a nozzle 1.4–1.6 mm.

⚠️ Attention: If you are thinning the soil solvent, never use 646th or 647th — they evaporate aggressively and cause microcracks. Only specialized thinners (PPG DT870).

FAQ: Frequently asked questions about cracks in the ground

Is it possible to paint over cracks in the ground without completely redoing them?

Possible, but only if there are cracks superficial (depth up to 30 microns). Algorithm:

  1. Sand out the defect P320–P400.
  2. Apply insulating soil (for example, PPG K36).
  3. Paint in 2-3 layers with drying between layers 10 minutes.

If the cracks are deep or go to the metal - only a complete rework!

How long after priming can I paint?

Depends on the type of soil:

  • 2K primers: 3–6 hours at 20°C or 30 minutes at 60°C.
  • 1K primers: 12–24 hours (air dry only).
  • Waterborne: 1–2 hours during forced drying.

Check readiness sticky tape: If no traces remain when peeled off, you can paint.

Why does the soil on plastic parts (bumper, spoiler) crack?

Plastic has a different coefficient of thermal expansion than metal. Solutions:

  • Use special primers for plastic (PPG DP60LF).
  • Apply elastic backing (3M™ Scotch-Weld™).
  • Dry at a temperature no higher 50°C.

On bumpers made of ABS plastic the soil cracks due to residual stresses after casting. Before priming, heat the part with a hairdryer until 40–50°C.

How to distinguish cracks in soil from cracks in paint?

Take the test:

  1. Carefully pry up the edge of the crack plastic pick.
  2. If it peels off color layer - a crack in the paint.
  3. If visible grey/beige layer - crack in the ground.
  4. If a crack deep and goes to the metal - the problem is soil adhesion.
Is it possible to drive with cracks in the ground?

Short-term (1-2 weeks) - possible, but:

  • ⚠️ Cracks accumulate moisture → risk of corrosion.
  • ⚠️ Paint around cracks starts to peel off.
  • ⚠️ At speed > 100 km/h cracks may expand due to wind load.

If there are cracks on thresholds or bottom — you can’t drive: this is a direct road to through corrosion!