Acid soil (aka phosphating or wash primer) is an indispensable material when preparing metal surfaces of a car for painting. It not only improves the adhesion of subsequent layers (epoxy primer, putty, paint), but also protects the metal from corrosion by forming a thin phosphate film on its surface. However, improper dilution of acidic soil can negate all its advantages: from poor hiding power to chemical conflict with other materials.

In this article we will look at how to dilute acidic soil. no lumps or errorswhat proportions to use for different brands (Reoflex, Novol, Body 960), and we will also talk about typical problems faced by both beginners and experienced painters. We will pay special attention compatibility of acid primer with epoxy compounds and acrylic paints - this moment is often missed, which leads to peeling of the coating after a few months.

Before you start practicing, remember: acid primer is applied only on bare metal (after sandblasting or grinding) and never Do not use over putty or old paintwork. Its task is to prepare the surface, not level it.

What is acid soil and why is it needed?

Acid primer is a two-component composition based on phosphoric acid, which reacts chemically with metal, forming a protective film. Unlike epoxy or acrylic primer, it does not just “stick” to the surface, but literally pickles top layer, creating microscopic pores for better adhesion of subsequent materials.

Main functions of acid soil:

  • 🛡️ Anti-corrosion protection — phosphates the metal, preventing rust even if the top layers of paint are damaged.
  • 🔗 Improved adhesion — without it, epoxy primer or putty may peel off after 1–2 years.
  • 🧪 Neutralization of oil and grease residues — dissolves microparticles that might remain after degreasing.
  • ⚡ Acceleration of drying - some brands (for example, Reoflex Wash Primer) polymerize in 15–20 minutes at room temperature.

Important: acidic soil does not level the surface and does not replace epoxy primer. Its layer should be minimal (10–15 microns), otherwise there will be problems with drying and adhesion. He also not compatible with polyester putties - they can be applied only after complete drying and neutralization of the acid layer.

⚠️ Attention: Acid soil is toxic! When working, be sure to use respirator with organic vapor filter (labeling A2P3), gloves and glasses. Phosphoric acid fumes irritate mucous membranes and can cause chemical burns.

Composition of acid soil: what is included in the kit

A standard kit for preparing acid soil includes:

  • 🧴 Base (base) - a liquid based on phosphoric acid and resins (usually sold in 1 liter cans).
  • 💊 Activator (hardener) - starts the polymerization reaction. Without it, the soil will not dry out.
  • 💧 Thinner - adjusts the viscosity for spraying. Can be included or purchased separately.

Some manufacturers (for example, Novol) supply soil in the form of a ready-made kit with measuring containers, others (like Body 960) require the purchase of an activator and diluent separately. Always check the instructions on the can - proportions may vary even for the same brand depending on the batch.

Component Purpose Approximate proportions
Base Contains phosphoric acid and binding resins 100%
Activator Initiates the curing reaction 10–20% of the base volume
Thinner Bringing to working viscosity 5–15% of the total mixture

Label note: Some acidic soils (e.g. PPG DP40/DP401) require pre-mixing the activator with the diluent, and only then adding to the base. Misalignment results in uneven curing and blistering.

📊 What acid primer do you use?
Reoflex
Novol
Body 960
PPG
Other
Didn't use

General formula for diluting acid soil:

Base : Activator : Thinner = 100% : 10–20% : 5–15%

But these numbers may vary! Below we give the exact proportions for popular brands (the data is current at the time of publication, but always check the packaging):

  • 🔹 Reoflex Wash Primer: base 100% + activator 15% + diluent Reoflex 420 10%. Drying time: 15–20 minutes at +20°C.
  • 🔹 Novol Wash Primer 486: base 100% + activator 20% + diluent Novol 486 Thinner 5–10%. It is demanding on temperature - it does not polymerize well below +15°C.
  • 🔹 Body 960: base 100% + activator 10% + diluent Body 710 15%. Suitable for operation over a wide temperature range (+10°C to +30°C).
  • 🔹 PPG DP40/DP401: base 100% + activator DP401 (25%) + thinner DX330 10%. Used in professional painting booths.

To measure components accurately, use measuring cups or electronic scales. Some masters pour by eye, but this is fraught with:

  • ❌ Excess activator → the soil dries too quickly, microcracks form.
  • ❌ Lack of activator → the primer does not polymerize and remains sticky.
  • ❌ Too much thinner → coverage decreases, more layers are required.

Thoroughly clean the metal from rust and oil | Degrease the surface Antisilicon or White spirit|Prepare measuring containers and mixer|Wear protective equipment (respirator, gloves, goggles)|Check the room temperature (optimally +18–25°C)-->

Step-by-step instructions for dilution and application

Diluting acidic soil is a simple process, but requires care. Follow this algorithm:

  1. Surface preparation. The metal must be clean, dry and free of grease. Use sandpaper P80–P120 to strip, then wipe White spirit or a specialized degreaser (Antisil, Prepsol).
  2. Mixing components. Pour the base into a clean container, then add the activator and thinner in the required proportions. Stir slowly (to prevent bubbles from forming) for 1-2 minutes.
  3. Viscosity check. The soil should flow from the stick in a uniform stream, without forming drops. If it is too thick, add another 1-2% thinner.
  4. Filtration. Strain the mixture through filter 125–190 µm (you can use a nylon stocking) to remove lumps and foreign particles.
  5. Application. Use spray gun with nozzle 1.3–1.5 mm and pressure 2–2.5 atm. Hold the gun 20–25 cm from the surface. Apply thin layer (10–15 microns) - the soil should not flow!
  6. Drying. At +20°C the soil sets in 10–15 minutes, complete polymerization takes 1–2 hours. Do not blow dry as this may cause uneven cure.

After the acid primer has dried, it can be coated with epoxy primer or putty (if leveling is required). But remember: acidic primer cannot be sanded - it's too thin. If you need to remove defects, it is better to apply another layer.

What happens if you apply acidic primer to putty?

Acidic soil will corrode polyester putties (e.g. Novol Plus or Body Standard), causing blistering and peeling. If you need to prime a putty surface, use epoxy primer or acrylic filler.

Common mistakes and how to avoid them

Even experienced painters sometimes make mistakes when working with acidic soil. Here are the most common:

⚠️ Attention: If after application the primer remains sticky for more than 30 minutes, this is a sign lack of activator or low temperature. Do not try to “dry” it with a hairdryer - remove the layer with a solvent and reapply.
  • 🌡️ Low temperature operation (below +15°C). Acidic soil does not polymerize well in the cold. Solution: use drying accelerator or move the work to a heated box.
  • 💦 Too thick layer. The soil does not have time to dry, and smudges form. Solution: Apply 1-2 thin coats at 5-10 minute intervals.
  • 🔄 Application over old paint. Acid primer only works on bare metal! Solution: remove the paintwork down to the metal with sandblasting or a grinder.
  • 🧴 Using the wrong thinner. For example, acetone or 646th solvent can roll up the soil. Solution: Use only branded thinners (Reoflex 420, Novol 486 Thinner).
  • ⏳ Interlayer drying time violation. If you apply epoxy primer too early, the acid primer will not have time to react with the metal. Solution: pause for 15–30 minutes (see instructions).

Another common problem is chemical conflict between acid and epoxy primer. To avoid peeling, use primers one manufacturer or proven combinations (for example, Reoflex Wash Primer + Reoflex Epoxy).

💡

If the soil begins to gel in the jar (clumps appear), do not try to “reanimate” it with solvent. This is a sign that the activator has already reacted. It's safer to throw away the can and open a new one.

Compatibility with other materials: what can be applied over

Acid soil is intermediate layer, and it always needs to be covered with other materials. Here's what you can apply over it (and in what order):

Material Compatibility Waiting time before application Notes
Epoxy primer ✅ Yes 15–30 minutes The best option for further processing. For example, Novol 420 or Body 989.
Acrylic primer filler ✅ Yes 30–60 minutes Suitable for smoothing out small defects. Example: Reoflex 2K Filler.
Polyester putty ⚠️ Conditional 1–2 hours Only after complete polymerization! Use putties marked "for acidic soils."
Acrylic paint (base) ❌ No — The paint will not lay flat without an intermediate layer of epoxy or acrylic primer.
Varnish ❌ No — The varnish is applied only to a fully prepared surface (primer + paint).

Important: if you plan to putty the surface after acid primer, use special putties with corrosion inhibitors (for example, Novol Polyester Stop or Body Light). Regular polyester putties can swell after a few months.

💡

Acid primer is not a final coating, but intermediate layer for metal preparation. It must always be sealed with epoxy primer or putty within 24 hours, otherwise it will lose its protective properties.

Safety precautions: how to work without harm to health

Acidic soil contains phosphoric acid and volatile solvents, which are dangerous to the skin, eyes and respiratory tract. Minimum protection set:

  • 😷 Respirator with class filter A2P3 (protection from organic vapors and dust).
  • 🧤 Nitrile gloves (latex ones are dissolved by solvents!).
  • 👓 Safety glasses with side splash protection.
  • 👕 Workwear with long sleeves or an apron.

Operating rules:

  • 🚫 Do not smoke or eat in the room where painting is being carried out.
  • 🪟 Provide supply and exhaust ventilation (or work outdoors).
  • 🧯 Keep a neutralizing solution (e.g. 5% soda solution) in case the soil gets on your skin.
  • 🔥 Avoid sources of open fire - solvent vapors are easily flammable.

If soil gets on your skin:

  1. Immediately flush the affected area with plenty of water.
  2. Apply a neutralizing solution (soda or soap solution).
  3. If there is redness or burning, consult a doctor.
⚠️ Attention: In case of vapor poisoning (dizziness, nausea), immediately go out into fresh air and drink water. If symptoms do not go away, call an ambulance. Don't self-medicate!

FAQ: Frequently asked questions about acid soil

Is it possible to dilute acidic soil without an activator?

No. Without an activator, the primer will not polymerize and will remain sticky. The activator triggers a chemical reaction that binds the soil components to the metal. If you forget to add activator, don't try to "fix" it after application - remove the layer with solvent and start again.

How long does diluted acid soil last?

The finished mixture (base + activator + diluent) must be used within 1–2 hours (depending on temperature). After this, it begins to gel and becomes unsuitable for application. Some brands (eg PPG DP40) have a “lifetime” of up to 4 hours, but this is indicated on the packaging.

Can acid primer be applied with a brush?

Technically possible, but not recommended. The brush leaves streaks and does not provide an even layer. In addition, acid primer is very liquid, and it is difficult to control the thickness with a brush. For small areas (such as welds), use mini spray gun or an aerosol can (if available in the manufacturer’s line).

How to remove acidic primer if it gets on glass or plastic?

Fresh drops can be removed White spirit or Solvent 647. If the soil is already dry, use special washer (for example, Body 700) or mechanical cleaning (blade, scraper). Use caution on plastic - solvents can damage the surface.

Is it possible to paint directly over acidic primer, without epoxy?

No, this is a grave mistake. Acid primer is not intended for direct contact with paint - it is too porous and does not provide the necessary adhesion. Always apply over it epoxy primer (1–2 layers) or acrylic filler. The exception is some one-component acid soils (for example, Reoflex Wash Primer in cans), but it is better to cover them with an additional layer.