When repairing a car body, choosing the right primer is 50% of success. A mistake at this stage will lead to corrosion, paint peeling or bubbles under the paintwork within a year. The two most popular types of soil are - acidic and regular (epoxy/acrylic) - solve different problems, but many masters confuse them or use them for other purposes.
In this article, we will look at how acidic soil differs from regular soil at the level of composition, properties and application. You'll know when you need it reactive primer, and where acrylic is enough, how to apply them correctly and what mistakes lead to defects. For clarity, we added a comparison table and a checklist for surface preparation.
1. Composition: chemistry versus mechanics
The main difference lies in the formula. Acidic soil (aka phosphating or reactive) contains orthophosphoric acid, which reacts chemically with the metal. As a result, a thin film of phosphates forms on the surface - it protects against corrosion and improves the adhesion of subsequent layers.
Composition of a typical acidic soil (for example: PPG DP40/DP40LF):
- 🧪 Orthophosphoric acid (10–30%) is a reagent for the formation of a phosphate layer.
- 🧴 Polyvinyl butyral (PVB) is a binder.
- 💧 Solvents (alcohols, ketones) - for quick drying.
- 🔬 Corrosion inhibitors - additional protection.
Regular soil (acrylic or epoxy) works on the principle of mechanical protection. It does not react with metal, but creates a physical barrier. It is based on:
- 🎨 Acrylic or epoxy resins are the basis for film.
- 🔄 Fillers (talc, mica) - level the surface.
- 🧴 Solvents - regulate viscosity.
- 🛡️ Anti-corrosion additives (optional).
Acid primer should not be applied to putty or old paint - it only reacts with bare metal. For these surfaces, use an acrylic leveling primer.
2. Purpose: when to use what
Acidic soil - this is the first and mandatory layer on bare metal, especially if:
- 🔧 You are repairing an area with rust or after welding.
- 🚗 You are processing a new part (for example, a wing or threshold).
- 🔥 You work with aluminum or galvanized steel (special acid primers are required, for example, 3M™ Scotch-Weld™ DP460).
Regular soil is used for:
- 🎨 Leveling the surface before painting (filler primer).
- 🔄 Intermediate layer between putty and paint.
- 🛡️ Additional protection in a multi-layer system (for example, epoxy primer under acrylic).
Critical error: apply acid primer over putty or old paint. It will not adhere to the surface, and the paintwork will peel off. For such cases you need adhesive primer (for example, Sikkens Autoclear Plus).
3. Application technology: step by step difference
The process of working with acidic and regular soil is radically different. Here are the key steps:
| Stage | Acidic soil | Regular primer (acrylic/epoxy) |
|---|---|---|
| Surface preparation | Degreasing + sandblasting or grinding to P80–P120 |
Degreasing + sanding under primer (P180–P240) |
| Number of layers | 1 thin layer (10–15 µm) | 2–3 layers (up to 100 µm) |
| Drying time | 5–15 minutes (at +20°C) | 30–60 minutes (acrylic) or 2–4 hours (epoxy) |
| Post-processing | Rinse with water (to neutralize acid) + dry | Sanding P320–P400 before painting |
Remove rust to bare metal | Degrease the surface Antisilicon|Apply the primer in one thin layer|Rinse with water after 10-15 minutes|Dry before applying the next coat-->
⚠️ Attention: If acid primer is not rinsed with water after application, acid residue will continue to react with the metal and destroy it. This is a common cause of through corrosion after 1-2 years.
4. Material compatibility
Acidic soil not compatible with the following materials:
- 🚫 Putties (polyester, epoxy) - acid eats them away.
- 🚫 Old paint or varnish - no adhesion.
- 🚫 Plastic and composites - a special primer for plastic is required (for example, PPG DP742).
Regular soil is more universal, but there are nuances here too:
- ✅ Acrylic filler primer can be applied to putty and old paint.
- ✅ Epoxy primer (U-Pol Acid #8) is suitable for aluminum and galvanizing, but requires a preliminary acid coat.
- ⚠️ Not all epoxy primers are compatible with waterborne paints - check the technical data sheet.
What happens if you apply acidic primer to putty?
After 3-6 months, bubbles under the paint will appear in this area, and the putty will begin to crumble. The reason is that the acid destroys the polyester base of the putty, and it loses its strength. It can only be corrected by completely removing the layers down to the metal and repeating the repair.
5. Pros and cons: what to choose for your case
Advantages of acid soil:
- 🛡️ Absolute corrosion protection — the phosphate layer blocks oxidation.
- 🔗 Excellent adhesion to bare metal (including aluminum).
- ⚡ Quick drying (5-15 minutes).
Disadvantages:
- 🚫 Does not level the surface - requires an additional layer of filler.
- 🧪 Toxic - work only with a respirator and gloves.
- 💦 Requires rinsing with water after application.
Advantages of regular soil:
- 🎨 Evens out defects (scratches, pores after grinding).
- 🔄 Universal - suitable for metal, putty, plastic.
- 🛠️ Easy to sand before painting.
Disadvantages:
- 🛡️ Weak anti-corrosion protection without a preliminary acid layer.
- ⏳Dries longer (up to several hours).
- 💰 Requires more layers for a high-quality result.
For maximum protection, use a combination scheme: acid primer (1 layer) → epoxy primer (1-2 layers) → acrylic filler (if necessary).
6. Popular brands and models
The following soils are most in demand among professionals:
Acidic:
- 🏆 PPG DP40/DP40LF — universal for steel and aluminum, dries quickly.
- 💎 3M™ Scotch-Weld™ DP460 - for galvanized surfaces.
- 💰 U-Pol Acid #8 - a budget option with good adhesion.
Conventional (acrylic/epoxy):
- 🎨 Sikkens Autoclear Plus — acrylic primer-filler with excellent hiding power.
- 🛡️ Spies Hecker Permahyd Hi-TEC 480 — epoxy primer for difficult conditions.
- ⚡ Mobihel Primer 2K — quick-drying acrylic primer (20 minutes).
⚠️ Attention: Cheap acidic soils (for example, "Auto repair" no brand) often contain an overly aggressive acid that corrodes the metal instead of protecting it. Check certificates and reviews before purchasing.
7. Common mistakes and how to avoid them
Even experienced craftsmen sometimes make mistakes when working with soils. Here are the most common:
- Applying acid primer to putty.
Consequence: paint peeling off after 3–6 months.
Solution: use an adhesive primer (for example, PPG DP742) or grind the putty down to metal in the application area. - No rinsing after acid soil.
Consequence: corrosion under the paintwork layer.
Solution: Rinse the surface with distilled water 10–15 minutes after application. - Using regular primer without acid on bare metal.
Consequence: rust will appear in 1–2 years.
Solution: Always apply acid primer as the first layer to the metal.
Pro tip: If you are repairing a small area (such as a chip or scratch down to metal), use acid primer in a can (for example, Motip Acid Primer). It is convenient for local repairs and does not require a spray gun.
FAQ: Frequently asked questions
Can acid primer be applied to galvanized steel?
Yes, but you need a special primer for galvanizing (for example, 3M™ Scotch-Weld™ DP460 or PPG DP60LF). Regular acidic primer can damage zinc coating.
How long should acid primer dry before applying epoxy?
Minimum 10–15 minutes at +20°C. After this, the surface must be rinsed with water and dried. Epoxy primer can be applied 30-60 minutes after washing.
Can I paint directly over acidic primer without filler?
Technically possible, but not recommended. Acid primer does not level the surface, and all defects (scratches, pores) will be visible under the paint. For high-quality results, use acrylic filler.
What primer is best for aluminum?
For aluminum the optimal combination is: acid primer for non-ferrous metals (for example, PPG DP460) + epoxy primer (for example, Spies Hecker Permahyd Hi-TEC 480). Aluminum oxidizes faster than steel, so an acid layer is required.
How to replace acidic soil if you don’t have it on hand?
As a last resort you can use epoxy primer with anti-corrosion additives (for example, U-Pol Acid #8 as the first layer). However, this is a temporary solution - after 1-2 years the risk of corrosion remains. For full protection, be sure to use an acidic primer.