What anti-corrosion paints are suitable for harsh weather conditions?

What anti-corrosion paints are suitable for harsh weather conditions?

Anti-corrosion paints are the first line of defense for steel against aggressive atmospheric factors. In harsh weather conditions—such as high humidity, frost, UV radiation, or salinity—choosing the right paint system determines the durability of the structure and its safety. Not every paint, even one labeled "for metal," can withstand the extreme environments of class C4 or C5. Therefore, the proper selection of coating type, application system, and substrate preparation is crucial.

What weather factors damage anti-corrosion coatings the most?

The resistance of anti-corrosion paint depends not only on its composition but also on the severity and frequency of external environmental impacts. Weather factors—humidity, temperature, UV radiation, pollution, and salt—are responsible for most cases of accelerated degradation of protective coatings. Combined with insufficient coating thickness and an incorrectly selected system, this creates a recipe for the need for rapid re-painting.

How do moisture, rain and condensation affect the development of corrosion?

Moisture is the main catalyst for corrosion. Water, especially water rich in salts, forms an electrolyte that triggers electrochemical processes leading to rusting. Even short-term condensation on a metal surface can initiate iron oxidation. Anti-corrosion paints must therefore have low water and vapor permeability and high adhesion to the substrate. In epoxy systems, this tightness is exceptionally high, which is why they are used as primers and intermediate coats.

What is the degradation of the coating under the influence of UV radiation?

Ultraviolet (UV) radiation destroys the chemical bonds in the coating structure. This manifests itself through chalking, color fading, loss of gloss, and microcracks. Epoxies (e.g., Telpox P100 , Telpox P170 ), despite their excellent mechanical and chemical resistance, are not very UV-resistant, so they always require a polyurethane (e.g., Telpur T300 ) or acrylic topcoat, or possibly a vinyl topcoat (e.g., Telkyd S220 ), which stabilizes the color and protects against photochemical degradation.

How do extreme temperature fluctuations and frost threaten steel?

Low temperatures cause materials to shrink, while high temperatures cause them to expand. Repeated cycles can lead to coating fatigue and microcracks, which allow moisture to penetrate the steel. Freezing water poses an additional threat – expanding in the cracks, mechanically cracks the paint. Therefore, paints used outdoors must remain flexible even at sub-zero temperatures, a feature achieved by polyurethanes (e.g., Krypton ProLine Silver TC ) and modern epoxy modifications.

How do salt (marine environment) and industrial pollutants accelerate rusting?

Salt (NaCl) is exceptionally aggressive – it penetrates micropores and acts as a catalyst for electrochemical corrosion. In marine environments, salt, moisture, and UV radiation interact, reducing the lifespan of coatings to several months if they are not properly selected. In industrial areas, the presence of sulfur and nitrogen oxides causes acid rain, which chemically degrades paint coatings. Epoxy-polyurethane and zinc paints with very low permeability are effective in this regard.

What types of anti-corrosion paints should you choose to protect steel?

The choice of paint type depends on the structure's operating conditions, exposure to external factors, and the expected durability of the system. Epoxy, polyurethane, alkyd, and zinc-rich paints are most commonly used, depending on the corrosion class.

When to use epoxy primers and intermediate coats?

Epoxy paints create a very tight, hard, and chemically resistant coating. They provide excellent protection against moisture, salt, fuels, oils, and industrial chemicals. Their disadvantage is their sensitivity to UV radiation, which is why they are used as primers or intercoats. They are used, among other things, on bridges, steel structures, tanks, and elements exposed to water. They are most commonly used in corrosivity classes C4 to CX. Examples of epoxy primers include: Telpox P100 , Telpox P170 , Telpox PM150 , and Neopox Primer 815 .

What are the advantages and properties of polyurethane topcoats?

Polyurethane paints are resistant to UV radiation, abrasion, impact, and weather conditions. They provide an aesthetically pleasing, smooth finish that retains its color and gloss for a long time. They are often the final layer of a paint system, combined with an epoxy base. Their flexibility allows them to compensate for thermal stresses, protecting the structure from cracking in both frost and heat. Krypton ProLine Silver TC and Krypton ProLine PU AL Primer special types of anti-corrosion polyurethane paints. These products not only retain their properties in harsh weather conditions, such as low temperatures and high humidity, but can also be applied in such conditions. Standard epoxy and polyurethane paints typically require above-freezing temperatures and low humidity, while Krypton ProLine can be applied even at -18°C and relative humidity above 90% (it is important that only the application surface is dry at the time of application).

Are traditional alkyd (phthalic) paints a good choice for exterior use?

Alkyd paints (e.g., Telkyd S200 TM ) and polyvinyl paints (e.g., Telkyd S220 ) are easy to apply and relatively inexpensive, but their resistance to weather conditions, including moisture, is limited. They perform best in environments with low aggressiveness (classes C1-C3), such as dry rooms, as well as urban environments, on roofs, and fences. They lose adhesion and deteriorate easily in harsh weather conditions, so they are not recommended for industrial or marine applications.

What are high-zinc primers and what is their role in cathodic protection?

High-zinc primers contain over 80% zinc dust. When applied to steel, they provide cathodic protection—zinc, being a less noble metal, corrodes instead of steel. These types of primers are sometimes used as the first coat in the most demanding systems, particularly in corrosivity classes C5-M and CX (marine and industrial).

How to choose an anti-corrosion system for the structure's operating environment?

The choice of paint is not only about aesthetics - it is primarily about adapting it to the specific environmental corrosivity class specified in the ISO 12944 standard.

What do the environmental corrosivity classes C1, C2, C3, C4, C5 mean according to the ISO 12944 standard?

The ISO 12944 standard defines six levels of environmental aggressiveness - from C1 (minimal) to CX (extreme):

• C1-C2 – dry, heated rooms, no condensation.
• C3 – urban and light industrial environments.
• C4 – industrial areas, coastal zones and warehouses with moisture.
• C5-M and C5-I – coastal facilities, ports, sewage treatment plants, shipyards.
• CX – extreme marine and tropical conditions.

Each class requires an appropriate combination of paints - e.g. epoxy + polyurethane or zinc + epoxy + polyurethane or zinc + epoxy + polyurethane.

How to interpret the expected system durability (low, medium, high)?

The ISO 12944-1 standard introduces four levels of durability:

• L (low) – up to 7 years,
• M (medium) – 7-15 years,
• H (high) – 15-25 years,
• VH (Very Long) – over 25 years.

In C4 and C5 environments, obtaining class "H", depending on the product selected, usually requires a coating thickness above 250 μm and a multi-layer system.

How to properly prepare a steel surface for painting?

Even the best paint will fail if the substrate is not properly cleaned. EN ISO 12944-4 specifies cleanliness levels for steel surfaces. In highly corrosive environments, preparation to Sa 2.5 (sandblasting for metal) is recommended. Degreasing and removing moisture before applying the first coat is also important. When applying renovation systems, for example, in a C3 environment, it is often sufficient to sand down well-adhered paint, degrease, and apply the next coat directly. It is important to remember that the adhesion of the topcoat will depend on the substrate preparation. Conversely, applying multi-layer systems over old coatings is not recommended due to lower adhesion and the simultaneous high stress on the coating resulting from environmental conditions.

What should you look for in the technical data sheet of an anti-corrosion paint?

The technical data sheet is a key document for the contractor - it specifies all the parameters that determine the durability of the system.

What does the recommended dry film thickness (DFT) mean and how can I control it?

DFT (Dry Film Thickness) is the thickness of a dry film, usually expressed in micrometers (μm). Too thin a layer will not provide a seal, and too thick a layer may crack. Measurements are taken with a magnetic-inductive meter or a painter's comb during painting (the technical data sheet often specifies the wet film thickness as the dry film thickness). Standard epoxy-polyurethane systems have DFTs ranging from over one hundred to even several hundred microns.

What information about application conditions (temperature, humidity) is crucial?

Epoxy paints do not tolerate application at excessively high humidity (above 80%) and low substrate temperatures. The surface temperature must be at least 3°C ​​above the dew point to avoid condensation. For polyurethanes, it is crucial to avoid moisture during curing – contact with water vapor can lead to blistering.

The exception are the previously mentioned special anti-corrosion paints: Krypton ProLine PU AL Primer and Krypton ProLine Silver TC , which can be applied from -18°C and at relative air humidity above 90% (the condition is a dry surface at the time of application).

What is the repainting time and why is it important to adhere to it?

The recoating time is the period within which the next coat can be applied without sanding. Exceeding this time can disrupt intercoat adhesion, leading to failure of the next coat. In practice, each paint has a minimum and maximum recoating time, depending on temperature, humidity, and layer thickness. After this time, applying another coat of anti-corrosion paint requires sanding the last coat before painting.

Summary

Selecting an anti-corrosion paint for harsh weather conditions requires knowledge of the environment, the structure's exposure, and the expected durability. The best results are achieved with a system combining zinc primer, epoxy, and polyurethane. The key to success lies not only in product quality but also in proper substrate preparation, adherence to application guidelines, and coat thickness.

What anti-corrosion paints are best for harsh weather conditions - frequently asked questions

Which painting system will best protect a steel structure in a coastal area (corrosion class C5-M)?

The best choice is a two-component system: epoxy primer + polyurethane topcoat. This system provides mechanical, chemical, and UV protection, ensuring the durability of the coating. Another option is a polyurethane system: polyurethane primer ( Krypton ProLine PU AL Primer ) + UV- and weather-resistant polyurethane topcoat ( Krypton ProLine Silver TC ).

Can a polyurethane topcoat be applied directly to an old, well-adhering alkyd coating?

This combination is not recommended, however, for lower corrosion classes, such as C1-C3, it can be attempted. Sanding and thoroughly degreasing the old surface is required each time.

What is the expected durability of a standard epoxy-polyurethane system in an industrial environment (C4)?

Depending on the selected products and thickness, you can choose a durability period ranging from short (up to 7 years), medium (up to 15 years), long (15 to 25 years), to very long (over 25 years).