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There are many factors that affect the recoating of coatings, mainly including the following categories.

The first one is the influence of additives. Like some silicon-based additives, such as defoamers for organosilicon, leveling agents for organosilicon, and some wax powders and pastes, they may migrate to the surface of the coating, forming a smooth oil film that is extremely difficult to adhere to, causing the new coating to shrink like water on a lotus leaf, forming shrinkage pores. There is no way to cover it evenly. Of course, we also need to eliminate a misconception that silicone will definitely affect recoating. For example, after adding organic silicon leveling agent, because its surface tension is relatively low, the low surface tension will migrate to the entire surface of the paint film, forming a monolayer.

When we reapply on this paint film, the leveling agent will still migrate to the new coating surface normally. Normally, it does not affect recoating. So under what circumstances will it affect recoating? It means that all or part of the organic silicon additives are interlayer. It will not migrate, either partially or completely.

For example, if the amount of organic silicon leveling agent added is too high, it only migrates a part, and there is still a part that cannot migrate in time. At this time, it may affect another situation, which is reactive organic silicon, which contains the leveling properties of the reactive groups and the crosslinking of the resin.

After being connected, if it is pulled by the bottom layer, it cannot be pulled upwards anymore. Of course, there is another situation, in high-temperature baking paint, for example, when we use polyether modified organic silicon leveling agents, these polyether modified organic leveling agents will decompose at high temperatures. After decomposition, they will generate hydroxyl groups, which can crosslink with amino groups and cannot migrate again, affecting the final recoating properties.

The second is that the internal stress of the coating can also affect the shrinkage of the recoated coating during the curing process. If the cohesion of the coating is too high, or the difference in thermal expansion system between the upper and lower layers of the coating is very significant, the internal stress generated at this time will offset or even destroy the adhesion between the layers. Causing the coating to wrinkle or peel off.

The third factor is that the wetting property of the topcoat can also affect recoating. If the surface elasticity of the topcoat is high and the surface elasticity of the primer layer is low, the topcoat cannot wet, spread and penetrate well on the primer, and it cannot be in close contact between the coatings, which will inevitably affect the adhesion between these layers,

The fourth one is related to the state of the primer layer. Has our primer layer been polished? What is its surface roughness? Has the polishing process affected the adhesion of the new coating? If the old paint film is too smooth or not polished, the new coating may be difficult to adhere to.

The fifth factor is that surface cleanliness may also affect interlayer adhesion, such as oil stains, dust, and release agents on the surface, which can significantly reduce its adhesion.

The sixth factor is that the aging degree of the coating can also affect interlayer adhesion, such as oxidation and powdering of the paint film, which can weaken the adhesion between layers.

The drying and curing state of the primer layer can also affect interlayer adhesion. For example, if the primer layer is not completely dry before reapplying, it can easily lead to white bubbles or poor adhesion. The drying time of the primer layer is too long, the paint film is completely hardened, the surface is too smooth and dense, and the density is high. There are fewer residual active groups than hydroxyl groups. The topcoat is also difficult to form chemical bonds with, resulting in a decrease in interlayer adhesion. In addition, during the construction of the wall, if the cement is not sufficiently cured and the alkalinity is too high, it can also damage the air film, thereby affecting the interlayer adhesion.

The eighth factor is that the temperature and humidity of the environment can also affect the adhesion between layers. When working in high humidity, rainy or foggy weather, moisture will condense on the surface of the coating, causing the new paint layer to turn white, reduce adhesion, and even react with certain components in the old paint layer. In special coatings, it can cause the precipitation of some salts. Low temperature can also affect the film formation of the coating and prolong its recoating interval.

The ninth factor is that the compatibility and compatibility of the coating system can also affect recoating, such as the presence of hydroxyl groups in the coating. Carboxyl groups, such polar groups, can form strong chemical or hydrogen bonds with similar groups on the surface of the bottom layer, which is helpful for enhancing interlayer adhesion. If the polarity of the upper and lower coatings does not match and non-polar topcoats are used on polar substrates, the adhesion will deteriorate. The matching coating system can also affect interlayer adhesion. Especially when water-based epoxy base is paired with water-based PU topcoat, problems are prone to occur when wet.

This is because the curing agent of waterborne epoxy primer is an amine based curing agent for waterborne PU, which is an isocyanate ester. Crosslinking occurs between these two curing agents, and their reaction rate far exceeds that of epoxy and amine, as well as hydroxyl and isocyanate esters. The reaction rate between them will result in insufficient cross-linking between the upper and lower layers of the interface layer, insufficient hardening at the interface, and no stress.

The adhesion between these layers will also become extremely poor. How to solve the problem of repainting? Firstly, we need to understand the factors that affect recoating, and then prescribe the right medicine. For example, if the use of silicone leveling agents affects recoating, we should try to use non silicone leveling agents as much as possible. If we must choose silicone leveling agents, we should also choose modifiers. Its terminal does not contain active groups, or has good temperature resistance, or some organic silicon leveling agents with good mobility. After applying the second layer, it will quickly migrate from the surface of the first layer to the surface of the second layer.

There is no thin film of leveling agent between the two layers, which has a relatively small impact on the adhesion between the layers. The use of organic silicon leveling agent should be minimized as much as possible, and the primer should be polished to remove any oil stains, mold removers, etc. on its surface as much as possible, which is also beneficial for the recoating process.

In addition, there are some leveling agents that can increase the surface tension of the paint film. Adding them to this primer can have some beneficial effects on interlayer adhesion, as well as some adhesion promoters. It can form chemical bonds between coatings, achieve mutual solubility and entanglement at the molecular level, and also increase interlayer adhesion. If the resin is lotion or resin containing polar groups, the introduction of hydroxyl carboxyl group in the acrylic paint can also significantly improve the adhesion between layers.