Common diseases of anti-corrosion paint and treatment plan

1. Flowing

When painting on a vertical surface, some of the paint flows under gravity, causing the thin film to develop uneven stripes and streaks. This is often due to slow solvent evaporation, applying too thick a layer, such as in heavily brushed areas, spraying too close, improper height, low paint viscosity, high solvent vapor content in the surrounding air, low airflow, or the complex geometry of the object being painted, which causes the paint to accumulate in gaps. To prevent sagging during application, it is crucial to strictly control the paint viscosity and enhance the operator's skill. Adjust the spray distance and avoid applying too thick a layer at once.

2. Bite the bottom

After applying the topcoat, the undercoat may be bitten or even peel off from the substrate, forming wrinkles. This is due to the solvent in the topcoat softening and swelling the undercoat. The main causes are improper matching of the undercoat and topcoat, or applying the topcoat too thickly before the undercoat is fully dry. It is important to choose the right paint and ensure the undercoat is completely dry before applying the topcoat. To prevent biting, apply the first coat thinly and wait a bit before applying the second coat.

3. Seepage color

The color of the primer or substrate is absorbed into the topcoat film, causing contamination. This occurs because the organic pigments or resins in the primer are dissolved by the solvent in the topcoat, allowing the color to seep into the topcoat. To prevent this color seepage, a layer of coating can be applied between the primer and the topcoat to isolate the primer with a tendency to penetrate.

4. White

This refers to the phenomenon where, after application, the paint forms a film that is matte, cloudy, or semi-transparent, and may even turn white during the drying process. This occurs due to a damp construction site with high moisture content in the air, rapid solvent evaporation, and a sharp drop in ambient temperature, causing water vapor to condense on the film, which can lead to the precipitation of resins or polymers, resulting in whitening. To prevent this, use organic solvents with high boiling points and slow evaporation rates, or preheat the surface to be coated (about 10℃ higher than the ambient temperature). Pay attention to the temperature, humidity, and solvent evaporation rate at the construction site.

5. Leave the oil and laugh

After construction, the paint film on the surface contracts, resembling water on wax paper, with spots and revealing the underlying layer. This is mainly due to the paint's poor surface wetting properties, which prevent it from forming a uniform film and instead causes it to contract and form droplets. Additionally, a thin paint film is also prone to contraction. Possible causes include the surface being too smooth or greasy, the paint not having been properly stored and aged, the paint being contaminated by grease or impurities, improper solvent evaporation, inadequate surface preparation, oil or water mixing in the spray gun's pipeline, low ambient temperature during application, or excessive paint viscosity.

6. Slow drying and re-adhesion

After applying the coating, if the film formation time exceeds the drying time specified in the product's technical conditions, or if the film does not cure, is hard, or is dry on the surface but not on the inside, these issues can occur. Re-adhesion occurs when the coating remains sticky after it has dried or cured. The causes of both problems are similar. Firstly, the quality of the coating is a concern; using solvents with slow evaporation rates or applying a too thick film can limit oxidation to the surface, preventing the bottom layer from drying completely. Therefore, the film should be applied thinly and evenly, even if it means applying several coats. If the primer is not fully dry, it can also affect the drying process, prolonging the drying time or causing re-adhesion. To address this, using solvents with faster evaporation rates, increasing the temperature of the application site, or adding additives can help.

7. Pinholes

On the paint film, needle-like small holes or pores similar to those in leather can form, with diameters of about 100um, known as pinholes. This condition is essentially similar to laughing, where some areas of the film are left unpainted during the formation process. However, pinholes penetrate directly to the surface of the material. In contrast, shrinkage leaves a very thin residual film. The primary cause of pinholes is the presence of air bubbles, weak pigment wetting, or an overly thin paint film. This issue can be attributed to both coating and painting techniques. To prevent pinholes, it is crucial to strictly adhere to the construction process, effectively addressing issues such as improper solvent selection and mixing, poor pigment dispersion, poor bubble release from the coating, unclean surfaces to be coated, and a poor painting environment.

8. Frothing

A portion of the paint film rises from the substrate or base layer, filled with liquid or gas, and circular protrusions appear on the film surface. Most cases of this issue are attributed to construction errors, primarily due to moisture or volatile liquids in the paint film. To prevent this, the surface to be coated must be clean, the paint film should be dry, and it should not be stored in high humidity environments. Additionally, porous primers should be sealed.

9. Tangerine peel

When spraying, if a smooth dry film cannot be formed and instead appears with an orange peel-like uneven texture, this phenomenon is known as orange peel. This issue is primarily caused by two factors: improper application techniques and the rapid evaporation of high-volatility components. To prevent this issue, you can use more diluents, preferably with solvents that have a higher boiling point. Adjusting the nozzle size, the distance between the spray gun and the surface being coated, and adding leveling agents can also help.

10. Wrinkle

Creasing occurs when the paint film, applied directly to the base layer or a dried primer, develops wrinkles during the drying process. These wrinkles appear as uneven, bumpy ridges due to the uneven drying of the inner and outer layers. This is often caused by an excess of drying accelerator, which causes the faster-drying surface to cover the slower-drying layer, leaving the slower-drying layer with no room to expand and causing it to contract upwards, resulting in wrinkles. Additionally, applying the paint film too thickly, the outer layer not drying properly, placing the paint under intense sunlight, or using excessively high baking temperatures can all lead to wrinkles. To address this issue, methods such as reducing the amount of drying accelerator, adding a slower-drying paint of the same type, and selecting appropriate solvents can be used. Furthermore, painting in a crisscross pattern and rapid temperature increases during baking can also cause this phenomenon.

11. Poor cover bottom, exposed bottom

After applying a coat of paint, the underlying layer can still be seen with the naked eye. If this occurs due to missed spraying or applying too thinly, it is referred to as 'exposure.' This issue is typically caused by insufficient paint coverage, low pigment content, unstirred pigment sedimentation, and overly thin paint. To address these issues, in addition to selecting the right paint, during application, ensure thorough mixing, careful application, and the use of paints with strong coverage.

12. Light loss

When the topcoat film dries, if it fails to achieve the desired gloss or if the gloss gradually decreases after several hours or weeks, this is referred to as gloss loss. The natural phenomenon of a coating's gloss gradually decreasing over time due to aging is not considered gloss loss. In terms of the coating itself, the issues often stem from an unsuitable formula, improper selection and mixing of pigments, inappropriate polymerization degree of resins, and poor mutual solubility. From the construction perspective, problems can arise from poor surface preparation of the object being coated, roughness of the coating surface, and excessive absorption of the topcoat. Climate factors also play a role; in cold weather, water vapor condenses on the surface, causing the coating to lose its gloss. For baked coatings, if they are placed in baking equipment too early, the coating film may not have enough time to level out, leading to pigment accumulation on the surface, forming pores, which appear as gloss loss when observed with the naked eye. To prevent this, carefully prepare the surface to be coated, ensuring it is smooth and free of gaps, strictly adhere to the specified drying conditions, and apply a corresponding sealing layer if necessary.

13. Powdering

Under the influence of climate, the paint film may develop surface damage or powdering as its gloss decreases. This typically appears white, and when the paint surface is touched, pigment particles can easily adhere to the fingers, a phenomenon known as powdering. This occurs because the paint film has been exposed to ultraviolet light for a long time, causing the paint to lose its adhesion near the pigment particles. The powdering effect is limited to the surface, with only a small amount of powdering at a time, and the underlying paint film can still remain intact until it is completely damaged. The degree of powdering depends on factors such as the type of base material, pigment variety, and the ratio of pigment to base material. External factors that contribute to powdering include the natural environment where the paint film is exposed, such as ultraviolet radiation, moisture, oxygen, marine climate, and chemical corrosion. If the paint film is too thin or is exposed to rain, fog, frost, or dew before drying, powdering can occur prematurely. Therefore, selecting the right type of paint is crucial. For example, epoxy coatings, which are resistant to corrosion, have poor weather resistance and can lead to early powdering. The same applies to asphalt paint. During application, it is important to ensure that the paint film reaches a certain thickness.

14. Cracking

The phenomenon of cracks appearing on the paint film is known as cracking. It can be categorized into fine, coarse, and crazing types, all of which are signs of the paint film's aging. Crazing refers to the paint film breaking through to the substrate, exposing the material or cracking to the point where it is not fully penetrating, resembling the patterns on a turtle's back. Most paint films will develop crazing after long-term use, indicating that the coating has failed and needs to be recoated, which is not considered a defect. Defective cracking refers to the appearance of crazing in the paint film shortly after application. This is usually due to the mismatch between the primer and the topcoat, such as applying a hard paint film over a long oil-based primer, which can lead to a lack of elasticity in the paint film, causing inconsistent expansion and contraction forces between the two layers. If the primer is not dry, the coating is too thick, and the topcoat is applied before it has fully dried, it is most likely to crack. Cracking caused by external factors can also be severe. High temperatures and humidity in tropical regions, where the paint film expands and contracts due to temperature changes, and frequent water absorption and evaporation, can easily lead to crazing. Additionally, if the paint is not mixed thoroughly before use, the original design formula is altered, or if indoor paint is used on metal surfaces outdoors, etc., can also cause crazing. The problem of fine and coarse cracks can be addressed by selecting a more resilient paint, ensuring that the film forms after application aligns with the surface's expansion and contraction forces. For crazing, it is particularly important to use a compatible paint.

15. Falling off

As the lacquer film cracks and loses its adhesion, it eventually detaches from the surface or separates from the primer, leading to two main issues: the film peeling off in small pieces or the entire piece peeling off. These issues are often caused by improper surface treatment, inappropriate primer selection (such as a primer film that is too hard, making it difficult for the topcoat to adhere, or a primer with too much gloss), construction errors, an overly thick lacquer film, or insufficient drying between layers of the lacquer film, especially when exposed to moisture. The entire piece peeling off may be due to the mismatch of two coats of paint, or contamination between layers, or severe powdering of the lacquer film.

16. Rust

Shortly after the black metal is coated, red streaks may appear under the paint film or rust spots may appear through the paint film. Initially, the paint film appears yellow, then it cracks, leading to pitting, pinholes, and under-film corrosion, collectively known as rusting. This is due to poor surface quality of the substrate, rust that has not been thoroughly removed, inadequate pre-painting treatment, incomplete phosphating treatment, or an incomplete coating, such as pinholes or missed areas. Poor rust resistance of the coating, a too thin coating, and pinholes between layers not being properly covered can also cause issues. Moisture and oxygen can penetrate, leading to electrochemical corrosion. The object to be coated must be thoroughly cleaned, and phosphating treatment should be performed if possible to ensure the integrity of the coating, aiming to coat both the inner and outer surfaces of the workpiece.