Frequently Asked Questions about Coating Process
Frequently Asked Questions about Coating Process
Hanging
When painting on a vertical surface, some paint may flow under the action of gravity. Causes uneven streaks and flow marks on the paint film. The reason is often that the solvent evaporates slowly and is applied too thickly, such as with heavy guns. The spraying distance is too close, the spraying height is improper, the viscosity of the paint is too low, the solvent vapor content in the surrounding air is too high, the air flow is too low, or the geometry of the object to be coated is complex, the paint accumulates in gaps, etc. In order to avoid sagging during construction, attention should be paid to strictly controlling the construction viscosity. Improve operator proficiency. Adjust the spray distance. One application should not be too thick.
Bite the bottom
After applying the topcoat, the lower coating layer may be bitten off or even separated from the substrate, appearing in a crepe-like pattern. This is caused by the solvent in the topcoat softening and swelling the primer. The main reason is that the primer is not properly matched or the topcoat is applied before the primer is dry or is applied too thickly. Pay attention to choosing matching paints and wait until the primer is completely dry before applying the topcoat. To prevent biting, the first coat should be applied thinly and wait for it to dry before applying the second coat.
Bleeding
The color of the basecoat or substrate is dissolved into the topcoat film, causing the topcoat to become stained. The reason is that the organic pigments or resins in the base coat are dissolved by the solvent of the top coat, causing the color to penetrate into the top coat. To prevent color bleeding, a layer of paint can be used to isolate the primer that has a tendency to bleed when applying topcoat.
whitish
This means that during the drying process after construction, the film of the hair paint will become matte, muddy, translucent or even white. The reason is that the construction site is humid, the atmosphere contains a lot of water, the solvent evaporates too quickly, the surrounding temperature drops sharply, and the water vapor condenses on the film, which easily causes the resin or polymer to precipitate and turn white. Preventive measures can include using organic solvents with high boiling points that evaporate slowly or preheating the surface to be coated (making it about 10°C higher than the ambient temperature). Pay attention to the temperature and humidity of the construction site and the evaporation rate of the solvent.
laugh
It means that the paint film shrinks on the surface after construction, just like water applied on wax paper, with spots and spots, exposing the bottom layer. The main reason is that this type of paint has poor surface wettability, which prevents it from forming a uniform film layer and shrinks in a bead shape. In addition, the thickness of the paint film is too thin and it is easy to shrink. The reasons may be that the coated surface is too slippery or greasy, the paint has not been stored and matured, the paint is contaminated by grease or impurities, improper solvent evaporation, improper surface treatment, spray gun Oil or water is mixed into the pipeline, the ambient temperature is too low during construction or the viscosity of the paint is too high, etc.
Slow drying and tackiness
The film formation time after paint application exceeds the drying time specified in the product technical conditions or the paint film does not solidify, has low hardness, and does not dry on the surface or inside, etc. Backtack is the phenomenon of grease still sticking to paint after it dries or solidifies. The reasons for both are similar. The first choice is the quality of the paint, the use of slow-volatile solvents, and the construction of a paint film that is too thick so that the oxidation is limited to the surface and the bottom layer cannot dry thoroughly. Therefore, the paint film should be applied thinly and evenly. It is better to apply more coats. If the primer is not completely dry, it will also affect the drying of the paint, prolonging the drying time or causing it to become sticky. This can be compensated by using fast-evaporating solvents, raising the temperature of the construction site, adding additives, etc.
Pinhole
Small needle-like holes or holes like leather or pores are produced on the paint film. The diameter of the holes is about 100um, which are called pinholes. The essence of this morbid condition is basically similar to that of laughter, in that some parts appear blank and unpainted during the film formation process. The difference is that the pinhole penetrates directly to the surface of the material. The shrinkage actually leaves a very thin residual film. The main cause of pinholes is the presence of air bubbles, poor pigment wetting or the paint film is too thin. There are coating reasons and painting reasons. The preventive measures are to strictly control the construction process and effectively eliminate factors such as improper selection and mixing of solvents, poor dispersion of pigments, poor bubble release of paint, clean surface of the object to be coated, and poor coating environment.
Blistering
A part of the paint film floats from the base plate or base coating, the interior is filled with liquid or gas, and a circular protrusion appears on the film surface. The cause of the disease is mostly attributed to construction errors, mainly the presence of moisture or volatile liquids on the paint film. The preventive measures are to clean the surface of the object being painted, to dry the paint film thoroughly, and to avoid placing it in a high-temperature environment. Porous primers, on the other hand, require sealing.
Orange peel
When spraying, the uneven dry film cannot form a smooth film but appears like an orange peel, which is called orange peel. There are two main forming factors; one is improper construction spraying, and the other is caused by high volatility and rapid volatilization. To prevent pathological conditions, you can use more diluent, preferably a solvent with a higher boiling point, adjust the size of the nozzle during construction, the distance between the spray gun and the object to be coated, etc. You can also add leveling agent appropriately.
Wrinkled
Wrinkling refers to the phenomenon of wrinkling during the drying process of the paint film applied directly to the base layer or on the dried base coat. Wrinkles are rough, shrunken, or heavily curved ridges. It is due to too much drier, which causes uneven drying of the inner and outer layers. The fast-drying surface covers the lower layer, leaving the lower layer with nowhere to stretch and has to gather upwards and wrinkle. Another example is that the paint film is too thick, dry on the outside but not dry on the inside, exposed to strong sunlight after painting, or grilled at too high a temperature, which can lead to wrinkles. It can be solved by reducing the drier, adding slow-drying paint of the same type, and selecting appropriate solvents. In addition, the painting must be spread vertically and horizontally, and the baking conditions may heat up too quickly, etc.cause this phenomenon.
The bottom of the cover is defective and the bottom is exposed
After applying a coat of paint, the underlying layer can still be seen with the naked eye. The phenomenon of bottoming out due to missed spraying or coating too thin is called bottoming out. It is usually caused by insufficient covering power of the paint, low pigment content, unstirred pigment precipitation, or too thin paint. To overcome the above disadvantages, in addition to the paint itself, attention should be paid during construction, stir thoroughly, apply carefully, and use paint with strong hiding power.
Lost light
After the topcoat film dries, it does not reach the desired gloss or the gloss slowly decreases after hours or weeks of painting, which is called gloss loss. This is not the natural phenomenon of exterior paints gradually losing gloss due to aging due to long-term exposure. When pathological changes occur, paint formulations are often inappropriate, and the selection of pigments is not properly dispersed and mixed. The polymerization degree of the resin is inappropriate, the mutual miscibility is poor, etc. The construction-related reasons include: poor surface treatment of the coated object, rough coating surface, and large absorption of paint on the surface. There are also climatic factors. During construction in colder weather, water vapor will condense on the film surface, and the paint will lose its luster. For baking paint, if it is put into the baking equipment too early, the paint film will often not have time to level, and the pigment will accumulate on the film surface, forming holes, which will cause the paint to lose its luster when viewed with the naked eye. Preventive measures are to carefully treat the surface to be coated to make it smooth and free of gaps, strictly adhere to the specified drying conditions, and apply the corresponding sealing coat when necessary.
Powdering
Under the influence of weather, the paint film will suffer further surface damage or powdery peeling as the gloss decreases. It is generally white. If you touch the paint surface with your hands, the pigment particles will easily adhere to your fingers. This is called powdering. This is caused by the paint film being affected by ultraviolet rays for a long time and the paint losing adhesion near the pigment particles. The change in powdering is limited to the surface, and the powder is removed very little each time. The paint film below can still remain intact until it is completely destroyed.
The degree of powdering depends on the type of base material, the type of pigment and the ratio of pigment to base, etc. The natural environment to which the paint film is exposed, such as ultraviolet radiation, water vapor, oxygen, marine climate, chemical corrosion, etc., are the external factors that cause the paint film to powder. If the paint film is too thin or if it is corroded by rain, fog, frost, or dew before it dries, powdering will occur in advance. Therefore, choosing the type of paint is very important. For example, as a corrosion-resistant epoxy coating, its weather resistance is poor and early powdering will occur. The same goes for asphalt paint. During construction, attention should also be paid to ensuring that the paint film reaches a certain thickness.
Crack
The phenomenon of cracks in the paint film is called cracking. There are fine cracks, coarse cracks, and cracks. These are all manifestations of the aging of the paint film. Cracks refer to the paint film being broken to such an extent that the surface is exposed or the surface is cracked but not visible to the bottom, just like the lines on the back of a turtle. Most paint films will develop cracks after years of use. This indicates that the coating has failed and needs to be recoated, and is not considered pathological.
Pathological cracking refers to the phenomenon of cracks in the paint film shortly after application. It is usually caused by mismatching of the primer and paint. For example, applying a hard paint film on a long-oil primer will cause the paint film to lack elasticity, resulting in inconsistent expansion and contraction of the two films. The primer is not dry and the coating is too thick. Applying the topcoat before it is completely dry will easily cause cracks. Cracking caused by external factors is also serious. Tropical areas have high temperatures and heavy humidity, and the paint film expands and contracts due to heat and cold. Water absorption, volatilization, and penetration are frequent and prone to cracking. In addition, the paint was not stirred evenly before use, which relatively changed the original paint design formula, and the indoor paint for metal surfaces was used for outdoor use, etc. The morbidity of thick and fine cracks can be solved by choosing a paint with greater toughness and achieving consistent expansion and contraction force with the surface after film formation. Pay special attention to the use of consistent paint for cracks.
Falling off
As the paint film cracks, it loses its adhesion, causing the film to separate from the surface or separate from the primer and fall off. There are two phenomena: falling off in small pieces or peeling off the whole piece. The causes are improper surface treatment, improper selection of primer (such as the primer film is too hard, making it difficult for the topcoat to adhere, or the primer gloss is too high), construction errors, the paint film is too thick or the layers of the paint film are dry and impermeable. Water vapor, etc. Peeling off in sheets may be due to the incompatibility of the two paints. Sometimes there may be contamination between the layers or the paint film may be excessively powdered.
Rusty
Shortly after the black metal is painted, red threads appear under the paint film or rust spots appear through the paint film. At first, the paint film is transparent and yellow, and then the paint film ruptures, pitting corrosion, needle corrosion and sub-film corrosion occur, collectively referred to as rust. . The reason is that the surface quality of the substrate is poor, the rust has not been removed, the pre-painting treatment is poor, or the phosphating treatment is incomplete, the coating is incomplete, such as pinholes, missing coatings, etc. There are also reasons such as poor rust resistance of the coating, the coating is too thin, the pinholes between layers are not staggered and covered, moisture, oxygen, etc. penetrate, causing electrochemical corrosion and other reasons. The object to be coated must be cleaned before painting, and phosphating treatment should be performed if possible to ensure the integrity of the coating, and strive to coat the entire internal and external surfaces of the workpiece with paint. htK
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