After local body repairs, the paint swelled bubbles after two weeks, although the technology seems not to have been broken, and this is a classic sign of ignoring the chemical inertia of the zinc coating. Galvanized metal It has high corrosion resistance precisely due to the protective layer, which, paradoxically, prevents the adhesion of paint materials without special preparation. Attempting to apply epoxy soil or acrylic filler directly to smooth zinc leads to the fact that the coating is laid down with a film without penetrating into the microstructure, and peels off under the influence of vibrations and temperature changes.
Many craftsmen mistakenly believe that it is enough to simply clean the surface to matteness, but mechanical action often only polishes zinc, making it even more slippery for chemical compositions. Galvanic galvanic It creates a low surface energy surface that requires the use of specialized primers or acid reagents to create roughness at the molecular level. If you miss the stage of proper preparation, even the most expensive auto enamel will not last long, and the repair will have to be remade completely, stripping everything to bare metal.
Causes of low adhesion and chemical properties of zinc⚠️ Note: Applying standard epoxy soil to fresh zinc without acid primer pretreatment guarantees coating detachment during the first year of operation.
The main problem lies in the chemical nature of zinc, which is an active metal and reacts with the components of some paints, forming zinc salts. These salts do not have adhesive properties and work as a separation layer between the body and paint. adhesion It is also disturbed by an oxide film, which is instantly formed in air, making the surface chemically inert.
For successful painting, it is necessary to use materials specially designed for such surfaces, or create conditions for mechanical adhesion. Acid soils (wash primer) contain orthophosphoric acid, which etchs the surface, creating micropores and providing chemical bonding. Ignoring this stage is tantamount to trying to stick Scotch tape to a greasy surface – the result will be predictably deplorable.
- 🔹 The chemical inertness of fresh zinc requires mandatory etching or the use of special primers.
- 🔹 The formation of zinc salts in contact with inappropriate paint components destroys the grip.
- 🔹 Smooth microstructure of the galvanic coating does not allow mechanically to catch conventional soils.
Selection of materials: soils and reagents for zincThe automotive chemicals market offers several solutions, and the choice of a particular product depends on the surface condition and the coating system planned. Jet soil (wash primer) is the most popular solution, as it simultaneously degreases, etches and creates an adhesive substrate. It is important to distinguish between single and two-component formulations: the latter provide more reliable protection and durability, although more difficult to prepare.
An alternative is special epoxy soils labeled "for galvanized surfaces", containing additives that improve adhesion. Such materials often require longer drying times, but create a strong insulating film to prevent electrochemical corrosion in the future. Acrylic filler soils It can only be used on top of a properly applied and dried reactive layer, as they do not hold on zinc by themselves.
Compatibility of materials
Can I apply epoxy to acidic acid?: Yes, but only after full polymerization of acidic soil (usually 30-60 minutes) and easy grinding. Direct application of epoxy to fresh acid is prohibited.
| Type of material | Appointment | Drying time (20°C) | Compatibility |
|---|---|---|---|
| Acid soil (Wash Primer) | Primary adhesion, etching | 15-30 minutes | Epoxy, acrylic soils |
| Epoxy soil (special) | Anti-corrosion protection, insulation | 12:24 hours. | Polyester putty, acrylic |
| Acrylic filler | Alignment, paint preparation | 3-4 hours | Basic enamel, varnish |
Surface Preparation: Mechanical ProcessingBefore applying any chemical compositions, the surface must be properly prepared mechanically. Abrasive processing It is necessary to remove the oxide film and create drawings for which the soil will catch. However, it is important not to overdo it: the use of too rough abrasive can leave deep scratches that will appear under the glossy varnish, and too small – just polish the zinc.
The optimal solution is to use P180-P240 grading abrasive materials for primary processing and P320-P400 for finish grinding before priming. Grinder. It should work at low revs so as not to overheat the metal, as heating can change the structure of the zinc coating and worsen adhesion. After grinding, the surface is necessarily blown with compressed air and degreased.
Use a silicone-based degreaser and two wipes: one apply the composition, the second (dry) immediately wipe the surface dry, not allowing the solvent to dry on its own.
- 🔸 Use abrasive P240 to create uniform mattage without deep furrows.
- 🔸 Avoid overheating of metal during grinding, work with short passes.
- 🔸 Carefully remove dust from pores and microcracks with compressed air before degreasing.
Technology of application of reactive soilApplication acid-soil This is a critical step requiring precise compliance with the proportions of mixing components and exposure time. Reagents are usually sold in two-component sets, where the base and activator must be mixed immediately before use. The viability of the finished mixture is extremely small - often only 15-20 minutes, so you need to cook just as much as you can apply.
Apply the material should be a thin, semi-wet layer, avoiding sweats and influxes, as a thick layer of acidic can remain sticky and not dry properly. Krascopult The air pressure must be adjusted to 2-2.5 atmosphere, and the spray torch is wide enough to cover evenly. After application, it is necessary to wait the manufacturer's specified time for the evaporation of solvents and the completion of the chemical reaction.
⚠️ Attention: Acid soil is hygroscopic and should not be left open in air for more than 6-12 hours (depending on the product) without overlapping with insulating soil, otherwise it will gain moisture.
☑️ Control of acid soil application
Main ground application and finishingAfter drying the reactive layer and easy re-insulation (if required by the instructions), the main insulating soil is applied. epoxy It lies on top of the acidic acid, creating a sealed barrier that cuts off the access of oxygen and moisture to the metal. It is this layer that carries the main protective function and provides the basis for subsequent materials.
On dried and polished epoxy soil, you can apply a polyester putty (if you need alignment) or immediately acrylic filler. Finishing paint is made according to standard technology: base-varnish or single-component enamel is applied to the prepared acrylic soil. It is important to observe interlayer exposures so that solvents do not boil under the layers of paint.
Acid soil is never ground to zero, its task is to create a chemical bond, not fill in irregularities. Only the upper insulating layer is grinded.
- 🔹 Epoxy soil provides maximum anti-corrosion protection.
- 🔹 Putty can be applied only to a completely dried and polished epoxy.
- 🔹 Finish enamel is applied to acrylic soil-filler for perfect smoothness.
Common Errors and How to Resolve ThemOne of the most common mistakes is to apply a thick layer of jet soil in the hope of improving protection, which leads to the opposite effect – the coating remains soft and does not dry. Disruption of technology
Another mistake is considered to ignore the temperature regime: painting at low temperature or high humidity can cause varnish clouding or poor polymerization of soils. Conditions of control In a paint chamber, it is as important as the right choice of materials. Do not save on quality primers, as the cost of rework always exceeds the price of good materials.
Mistake.
Reason.
Consequence
Decision
Bloating paint
Absence of acid soil
Loss of adhesion, corrosion
Complete sweep, primer application
Sticky layer
Too thick layer of acid
Inability to paint
Washing, re-application with a thin layer
Craters on paint
Bad degreasing
Defects in appearance
Grinding, re-greasing
FAQ: Frequently Asked Questions
Can I paint the galvanizing without removing the zinc layer?
Yes, you can and should. Removing the zinc layer mechanically to iron is not recommended, as you lose your corrosion protection. The main thing is to properly prepare the surface and use special soils.
How long after the acid soil can you paint?
Depending on the product, but usually the minimum time is 30-60 minutes at a temperature of 20 ° C. The maximum time of "life" of the open acid soil before the overlap is up to 12 hours (check the TDS of the specific manufacturer).
Will the usual epoxy soil be suitable for galvanizing?
Ordinary epoxy soil may not provide proper adhesion. You need either special epoxy soils marked "for zinc", or the use of acid primer as the first layer under the usual epoxide.
Do I need to grind the acid soil before applying epoxy?
In most cases, light refracting (e.g., with a P400-P500 abrasive) is recommended to remove possible dust and create a microrelief, but without breaking through the layer to the metal. Some manufacturers allow the application of "wet wet" in a certain time interval.
How long after the acid soil can you paint?
Depending on the product, but usually the minimum time is 30-60 minutes at a temperature of 20 ° C. The maximum time of "life" of the open acid soil before the overlap is up to 12 hours (check the TDS of the specific manufacturer).
Will the usual epoxy soil be suitable for galvanizing?
Ordinary epoxy soil may not provide proper adhesion. You need either special epoxy soils marked "for zinc", or the use of acid primer as the first layer under the usual epoxide.
Do I need to grind the acid soil before applying epoxy?
In most cases, light refracting (e.g., with a P400-P500 abrasive) is recommended to remove possible dust and create a microrelief, but without breaking through the layer to the metal. Some manufacturers allow the application of "wet wet" in a certain time interval.