Anti corrosion coating is widely used in metal industry such as architecture, furniture, automotive, ship and aircraft etc. It’s crutial process for surface protection in different working conditions. There’re lots of anti crossion coatings for various application, normally available as single component and two component coatings. In this article we’ll generally introduce the technical process and precautions of application.
What should you pay attention to when spraying single component coating?
The use of single component coating is relatively straightforward, such as alkyd paints, chlorinated rubber paints, acrylic paints, and the like. Single-component coatings can also be used after opening the barrel. Stirring is the minimum requirement. Below the primer, anti-rust pigments and fillers may precipitate and separate from the resin. For topcoats, the color may not bloom. Evenly. By mechanical agitation, the resin and the pigment and the filler can be uniformly mixed, and the color tone of the top coat can be more uniform.
Solvent coating such as oil-based anti-corrosive paint, alkyd paint and phenolic paint etc. the most important principle for these coatings is not to spray too thick. If the required paint film thickness is thick, it can be applied several times. In fact, these coating products can only achieve the best results after multiple constructions. This is because after they are applied, they react with oxygen in the air. If the paint film is too thick and the surface layer is dry, oxygen cannot penetrate deep into the interior, which will cause insufficient curing of the inner coating and affect its durability and hardness. The most obvious defects in coatings are wrinkles, cracks, and the like.
Another construction property of the oily coating is that its construction performance is quite good because it has better lubricity than other coatings, which makes it better to use brushing method. Solvent coatings have good surface penetration and can fully wet the surface. The biggest feature of chlorinated rubber coatings and acrylic coatings is that they dry quickly. Because of thermoplastic coatings, the coating will become soft when the temperature exceeds 40°C. This is of course not a problem during construction. The problem is if there is a dirty floating sediment in air, it will cause contamination on of coating surface and will not be easily removed.
What should I pay attention to when applying two-component paint?
Epoxy and polyurethane coatings are the most commonly used two-component coatings, the most important of which is paint mixing. These two components are usually divided into a base material and a curing agent, and a chemical reaction occurs after mixing to form a coating film to have a corresponding performance. If it is not well mixed, the chemical reaction will not be sufficient for guaranteed performance.
Two-component coatings must be thoroughly stirred and mixed by mechanical agitation. Remember not to use a wooden stick to mix the coating. Many curing agents are colorless and transparent, so it is difficult to tell if the curing agent is well mixed with the base. In this case, the use of power tools is particularly important. To solve this problem, some coating products are designed as colored curing agents so that they disappear after being evenly mixed. If a small amount of protrusion work such as repair or precoating does not run out of paint at one time, the remaining paint must be sealed. If the base component is not sealed, the solvent will run out and the impurities will fall in; if the curing agent group is not well sealed, it will affect its performance.
Why can’t the paint in the drum be stirred with a wooden stick?
Two-component coating products, such as epoxy coatings and polyurethane coatings, are usually packaged and packaged into component A (base) and component B (curing agent). The two components are packaged in a certain ratio. For the base A component, good mechanical agitation is required before mixing. Some builders are used to mixing with wooden sticks for convenience, rather than mechanically stirring with power tools. Due to the high level of assault solids in modern coatings, some are up to 80%, while others are solvent-free coatings. If the human body is stirred by a wooden stick alone, it is impossible to uniformly mix the two components, which may result in insufficient curing, and also reduce the chemical resistance and water resistance of the coating film.
What is the concept of maturation time or boot time when two-component paint is applied?
The epoxy resin product also stipulates that the inductin time after mixing is sometimes translated into the lead time. This time requires two components to be chemically reacted prior to construction before they can be constructed for optimum performance. The temperature will affect the curing time. The higher the temperature, the shorter the curing time. A poly cured epoxy resin coating with a curing time of 15-30 minutes (23℃).
What are the points of attention when using acrylic polyurethane topcoat during construction?
Acrylic polyurethane top coat is the first choice for modern heavy-duty coating and is widely used. It has its own special construction requirements compared to other two-component top coats.
As a curing agent isocyanate component, it is very sensitive to moisture. The lid must be sealed and stored in a dry place. Otherwise, it will cause moisture to enter and deteriorate. In particular, when paying a small amount of paint, pay attention to this when using different colors. It should be noted that in the atmosphere containing sulfur and chlorine, some lead-free red and yellow are easily discolored. At temperatures above 100 degrees Celsius, the paint film will be slightly discolored, especially white. To maintain long-term color retention, do not use temperatures above 80 °C. Conventional air spray results in better gloss and appearance. When using airless spray, it is recommended to add 5-15% thinner to ensure film formation, and sometimes it may be necessary to add 20%. Spray a layer of mist first, and after 2-15 minutes, spray it to the complete paint film to achieve the best film formation.
The relative humidity during construction is limited to 85%, but in order to avoid loss of light, it is best to control within 80%, the construction temperature can be as low as -5% or -10% °C. But when below 5 °C to achieve a hard dry film, it requires longer curing time. Acrylic-modified polyurethane top coats do not as fast as polyester-modified topcoats to achieve a hard paint film that appears soft and thermoplastic. If the adhesion is to be tested, it may not be ideal, but in the following weeks It can reach full cure within 30 days. Since the isocyanate component is very sensitive to moisture, in addition to controlling the relative humidity during construction, it is also necessary to predict the weather. For example, condensation or rain may not occur soon, or the paint film may lose light or even foam.
The more important feature of acrylic polyurethane topcoat is that it does not have the maximum recoating interval when it is overcoated. Due to the relatively low degree of cross-linking of acrylate, it can be well coated even after a long period of aging. Focus on. Of course, this adhesion is unlikely to reach the strength when recoated in 24H.
What should I do if the two-component coating is applied to metal surface without adding a curing agent?
Two-component coatings, such as epoxy coatings, polyurethane coatings, etc., sometimes due to worker error, sprayed onto the surface without adding a curing agent, what should be done?
Some people think that adding one or two diluted curing agents to cover a normally adjusted coating can basically solve this problem, and also design the ratio of diluent to curing agent and spraying procedure. In fact, this is a way of deceiving yourself and others. It is impossible to completely penetrate into the interior of the coating for the addition of the curing agent, not to mention the uniform penetration and then full curing. Therefore, after spraying a two-component coating without a curing agent, the only feasible method is to remove the uncured coating and then spray the normally mixed two-component coating.
Can inorganic zinc silicate coating be polished to the surface of ST3?
Partial repairs are inevitable during the construction of inorganic zinc silicate coatings. According to the general repair method, it is locally polished to ST3 grade, and then the paint is applied by brush or spray. However, inorganic zinc silicate coatings are not suitable for repair on polished surfaces. Experience has shown that the adhesion of such coatings is not good and will soon return to rust. Therefore, the repair of inorganic zinc silicate is preferably carried out by sandblasting to achieve Sa2.5 and has a certain roughness, and is not suitable for painting on the polished surface. (ST2 thorough manual and power tools for rust removal without observing, the surface should be free of visible grease and dirt, and there is almost no adhesion to the scale, rust, paint coating debris.) (St3 Very thorough manual and power tools are rusted with St2, but the surface treatment is much more thorough, the surface should have the luster of the metal substrate) (The surface should be free of visible grease and dirt when viewed without magnification, and No oxide scale, rust, paint coating and foreign matter. Any traces of residue should be only spotted or streaked with a slight stain.)
In multi-coat systems based on inorganic zinc silicate paint, it is often necessary Partially sand or sandblast it and then repair the primer. It is best to avoid using inorganic zinc silicate as a repair primer. Because it is naturally sprayed onto the surrounding organic coating when it is sprayed, it causes adhesion problems. The solution is to use an epoxy rich zinc material. A typical matching scheme is: inorganic zinc silicate primer 80μm. epoxy sealing paint 30μm. thick paste epoxy cloud iron intermediate paint 100μm. acrylic polyurethane topcoat 60μm. steel surface finish after the construction of this coating system, due to the end of the period Destruction and local rust, or in the future structural maintenance, if not all sandblasting out to Sa2.5, but only partial blasting or partial grinding to Sa3 level, then the maintenance of this time can not use the original system Inorganic zinc silicate primer, in this case the use of epoxy zinc-rich primer is the best choice, no need to use sealer,
Therefore the patching scheme is as follows:
- Epoxy zinc-rich primer 80μm
- High-build epoxy iron-iron intermediate paint >120μm
- Acrylic polyurethane topcoat 60 μm
What are the requirements for mixing and stirring inorganic zinc silicate coatings?
Inorganic zinc silicate coatings are usually two packages of zinc powder and liquid components. While mixing, the zinc powder is slowly added to the liquid component and stirred at the same time. Do not reverse the operation, adding the liquid component to the zinc powder can easily lead to agglomeration, and can not be stirred evenly. After mixing and stirring uniformly, it was filtered with a 60-80 mesh filter and used. While spraying, keep stirring to prevent precipitation of zinc powder. After mixing, it is required to use up within a certain period of time. This time is called the time after mixing. If it exceeds this time, since the viscosity of inorganic zinc silicate changes very slowly, the paint should not be used even if it has not changed, otherwise it will cause the film. Cracking.
What are the requirements for the construction method of inorganic zinc silicate coating?
Both air and airless spray can be used for the construction of inorganic zinc silicate. Roller coating is not suitable, and brushing is only used for small area repair. The construction speed of air spraying is slow. It should be noted that many builders use suction-type air spraying tools, which are not suitable for spraying inorganic zinc silicate primers. It is large and does not easily reach the specified dry film thickness. The recommended air spray equipment is a pressure tank. The pressure tank can hold 20L of paint. The compressed air is used to drive the paint out of the muzzle. The spray gun of this kind of equipment can be freely and flexibly sprayed because it is not restricted by the painted small pot, and the amount of paint mist is small, and the paint time is not required to be added frequently, which shortens the construction time.
If airless spraying is used, it must be noted that the pressure of the spray should not be too large, and the control should be between 110=150KGF/cm2 (depending on the material and viscosity), otherwise the atomization will be excessive due to excessive pressure, and dry spray, etc issues.
How to judge the curing of inorganic zinc silicate film?
The curing of inorganic zinc silicate coatings relies on relative humidity and temperature. Conventional inorganic zinc silicate coatings require about 24-72H for curing at 20 °C and RH (relative humidity) of 65-75%, and the curing time after adjustment is 16-24H, so the curing time of products varies from manufacturer to manufacturer. Please pay attention to the product description during construction. In low temperature and low humidity environments, the curing time of the paint film will be extended, the relative humidity should be kept above 65%, and the lowest temperature can be as low as -10 °C. Spraying water can solve the curing problem at low humidity. After the construction of 2-4H, you can spray clean water to keep the paint surface moist and help the curing.
The detection method of inorganic zinc silicate paint film curing method is MEK test method. According to Astm4752, the white cotton cloth is rubbed with MEK reagent and rubbed back and forth 50 times. If the color change is serious, it indicates that the curing has not been completed.
How much is the control of the inorganic zinc silicate paint film when spraying? What are the tips for spraying?
The film thickness of the inorganic zinc silicate coating is usually 60-80 μm in the multi-coating heavy-duty anti-corrosion system in China as a primer. Excessive dry film thickness can cause cracking of the paint film, and it is generally considered safe to be 150 μm or less. The cracking of the paint film is only due to the shrinkage of the paint film during drying and solidification, and it is also called mud crack due to the cracked shape like the muddy land which is dried and cracked under the scorching sun. The reasons for cracking are as follows:
- the paint film is too thick;
- the surface roughness is too low, especially on the weld, due to the high hardness, sandblasting is also difficult, resulting in insufficient roughness;
- relative humidity When the ventilation is too high, the surface layer will cure too quickly, which will also cause the film to crack.
- The composition of the paint is not uniform due to the precipitation of zinc powder. In actual construction, cracking problems are easily generated at the corners of the structure. When in order to achieve the minimum film requirements, the painter will take a few more shots in the corner, which leads to the film is too thick, and the corner film shrinks unevenly, it is easy to form a mud crack. For complex structures, small nozzles are often used, which in turn causes multiple shots to be sprayed, which can cause problems to get worse. Therefore, spray at the inner corner, you can use a large nozzle, spray up to two times, do not spray construction.
What kind of paint can be sprayed directly on galvanized steel?
Many coatings can be used on galvanized parts, and sometimes this choice depends on the method of surface treatment. However, some coatings cannot be used for galvanized parts, especially oily alkyd paints. There are many types of epoxy paints, they must be used exclusively for galvanized parts, usually with polyamide epoxy coatings. Epoxy coatings will be powdered in the sun, so topcoats should be coated with an aliphatic polyurethane topcoat that looks good and resistant to chemicals.
Waterborne acrylic coatings dry quickly, are good topcoats, and are easy to apply. However, water-based latex paints dry quickly, but it takes 2-4 weeks to fully cure. Therefore, in the initial construction of the structure, handling damage may occur. Chlorinated rubber and vinyl coatings have been used in galvanized parts in the past, but the Environmental Protection Act has provisions on VOCs that have limited their use. The surface of the galvanized parts after sandblasting and phosphating primers has good adhesion.
Acrylic paint can be applied directly to the surface of galvanized parts, of course, it is better to have a phosphating low paint base. Some acrylic paints contain ammonia, and if the HP value is high, the coating will peel off. If the galvanized part is buried underground, it can usually be coated with epoxy asphalt or pure oxygen coating, and it is best to apply it on the surface of the sandblasting or phosphating primer. Epoxy zinc-rich paints can usually be applied directly to the surface of galvanized parts, but not all epoxy zinc-rich paints are available. Please check the supplier’s coating product description. If sandblasting is used to treat the surface, it must be painted within 24H as the fresh zinc layer begins to oxidize. Applying a layer of phosphating primer or acrylic acid purifier first gives us enough time to apply.
Is the description a shelf life or a mixed use time?
In instruction manual, especially the two-component coating, the concept of the filling life is given, which refers to the time that a two-component coating can be used after mixing. For the sake of understanding, some people translate it into mixed use time. After this time, the coating will gel and cannot be used. The nature of the coating itself and the choice of curing agent have a large impact on the life of the coating. Filling life of different epoxy coatings:
- Epoxy coating category Filling life (23 °C)
- Polyamide curing agent epoxy coating 8H
- Amine adduct curing modified epoxy coating 2H
- Cashew nut shell oil cured epoxy coating 2H
- Solvent-free epoxy coating 30min
