How to select and combine anticorrosive coatings: how to match primer, intermediate coat and topcoat
Selection problems usually arise not from choosing the wrong paint model, but from three mismatches:Corrosion categorywhether an intermediate coat should be applied,Resin systemwhether intercoat compatibility is achieved,Service conditionsand whether (temperature/moisture/exposure) exceeds the service range of the paint. The following sections address these three points, followed by the three most common misjudgments on site.
Table of contents on this page
- How to build the coating system(8 items)
- Can different coatings be applied over each other(5 items)
- When to change the coating system for the service conditions(5 items)
- Others(12 items)
How to build the coating system
- What paint is used for steel structure corrosion protection?
- The most common three-coat system is epoxy zinc-rich primer + epoxy micaceous iron oxide intermediate coat + polyurethane topcoat (outdoor weathering) or fluorocarbon topcoat (strong color retention). For low corrosion categories and low DFT requirements, the intermediate coat may be omitted, but zinc-rich primer cannot be used alone as a topcoat.Related models:jotun-barrier-75-cn
- What is the function of epoxy zinc-rich primer?
- Two functions: zinc powder provides cathodic protection to the steel substrate (scratches are less likely to rust from the edges), and it provides an adhesion base for subsequent coatings. Its weathering resistance and barrier properties are limited, so an intermediate coat or topcoat must be applied over it.Related models:jotun-barrier-75-cn, hempel-1536c
- What is the function of epoxy MIO intermediate coat?
- Build total DFT, improve barrier properties, and bond primer and topcoat. Micaceous iron oxide is a lamellar pigment; stacked layers extend the diffusion path for water and oxygen. Thick-film build relies mainly on this coat; when a single coat cannot achieve the thickness, apply in separate coats according to the recoat interval.
- What type of paint is epoxy MIO intermediate coat?
- It is an epoxy anticorrosive intermediate coat, two-component (base + hardener), crosslinked by epoxy resin, and is not normally used directly as a topcoat. After curing, it has good water and chemical resistance but poor outdoor weathering, so it is generally not used directly as a topcoat.
- Can the epoxy MIO intermediate coat be omitted?
- Depends on design DFT and corrosion category. For indoor or general atmospheric environments with low total DFT requirements and C3 and below, primer plus topcoat can complete the system; for C5 and above (coastal high salt spray, chemical atmosphere), retaining the intermediate coat is recommended; otherwise DFT and barrier properties are difficult to achieve.
- What is the zinc content standard for epoxy zinc-rich primer?
- Standard systems classify zinc powder content differently (e.g., the ISO 12944 series has classifications for zinc-rich primers). The specific content is subject to the TDS and product standard for that model; products of different classes cannot be substituted for one another.Related models:hempel-1536c
- Can a one-coat primer-topcoat replace 'primer + topcoat'?
- For thin-film, low-corrosion-category indoor environments, yes; for heavy-duty corrosion protection, not recommended—one-coat primer-topcoat generally cannot cover C4/C5 requirements in terms of DFT and barrier properties. It also cannot replace the cathodic protection function of zinc-rich primer.
- Can epoxy zinc-rich primer be applied to hot-dip galvanized or cold galvanized surfaces?
- Yes, but this is 'zinc over zinc'; adhesion must be ensured by surface preparation: galvanized surfaces require sweep blasting or a dedicated treatment, followed by a compatible primer. Direct painting tends to cause large-area detachment, with high rework cost.
Can different coatings be applied over each other
- What is the difference between polyurethane topcoat and epoxy topcoat?
- Epoxy topcoat has better water and chemical resistance but chalking and gloss loss outdoors; polyurethane topcoat has good gloss and color retention and is suitable for atmospherically exposed areas. Outdoor steel structures generally use polyurethane or fluorocarbon topcoat.
- What is fluorocarbon topcoat, and how does it compare with polyurethane topcoat for selection?
- Fluorocarbon topcoat uses fluororesin as the main film-forming material; weathering and color retention are better than polyurethane, and the price is higher. For bridges, towers, landmarks, and other areas requiring long maintenance-free periods, select fluorocarbon; for general factory steel structures, polyurethane is sufficient.
- Can paints from different brands be applied over one another?
- Yes, but compatibility must be judged by resin system, not brand alone. The most reliable method is to check the 'compatibility/apply over' section in both TDS documents, or conduct a small-scale test before large-area application.
- Can epoxy and polyurethane be mixed and used as one coat?
- No. The two have different curing mechanisms; mixing results in incomplete crosslinking, causing non-drying, softness, or reduced adhesion. Epoxy cures by amine crosslinking, polyurethane by isocyanate reaction; when mixed, crosslinking is incomplete. Problems usually appear after several days, and the rework area is larger.
- Can new paint be applied directly over old paint film?
- First determine the old paint type and condition: the same system can be abraded and recoated; different systems (e.g., epoxy over alkyd) tend to cause lifting. Peeling and chalking old film must be removed to a sound layer. Peeling and chalking old film must be removed to a sound layer; otherwise the new paint will fail together with the old film.
When to change the coating system for the service conditions
- What system is specified for coastal high-salt-spray environments?
- Treat as C5: zinc-rich primer + epoxy MIO intermediate coat + weather-resistant topcoat (polyurethane or fluorocarbon); total DFT is determined by the design documents and the durability class of ISO 12944.Related models:jotun-barrier-75-cn
- What paint is used for high-temperature areas?
- Classify by temperature: silicone heat-resistant paint, phenolic epoxy, and inorganic zinc-rich each have applicable temperature ranges. Standard epoxy cannot replace heat-resistant paint. Before selection, confirm whether the actual operating temperature is constant or peak; the primer system must also be selected for the same temperature range.Related models:kansai-thermo-600-primer, kansai-thermo-600-finish
- What is the temperature resistance of epoxy zinc-rich primer?
- Standard epoxy systems are not suitable for long-term high-temperature service; the upper temperature limit is subject to the TDS for that model and must not be applied across models. For temperature-resistant applications, select a heat-resistant system, and select the entire system (primer, intermediate, topcoat) for the service temperature.
- Can paint be applied directly to a damp surface?
- Standard epoxy is not suitable and requires a dry surface. For damp or underwater conditions, use moisture-curing or high surface-tolerant products and state this clearly in the coating system. Pay particular attention to substrate condensation in coastal areas and where heavy dew occurs at night; moisture-curing products must also be specified in the coating system.
- Which is applied first, fireproofing coating or anticorrosive coating?
- Generally apply the anticorrosive primer first (fireproofing coatings have requirements for substrate rust prevention), then the fireproofing coating, and finally a topcoat as required. Follow the fireproofing coating manufacturer's system instructions for the specific sequence. The application windows of the two products must be coordinated; applying fireproofing coating before the anticorrosive primer is fully dry is a common failure point.
Others
- What project experience is required for thermal insulation and anticorrosion bids for thermal power plants?
- The common requirement is at least two thermal insulation and anticorrosion projects in the past three years with a contract value of not less than RMB 2 million and units of 300 MW or above. Qualification requirements also include Grade 2 waterproofing, anticorrosion and thermal insulation contracting, formwork and scaffolding specialty contracting, and a safety production license. Specific requirements follow the tender documents; the original text prevails.
- Can volume solids and weight solids be converted directly?
- No; the two require density for conversion. Wet-film consumption calculations use volume solids; within the same brand, zinc-rich primer and polyurethane topcoat values can differ substantially. Use only the row for that model in its TDS; estimating by experience will inevitably deviate.
- For epoxy that cures at -5 °C, will it dry readily at 5 °C?
- Two separate issues. Curing capability is only the manufacturer's limit and does not mean easy application: the -5 °C grade has a minimum recoat interval of 48 h, but hard dry requires 60 h, so the two values are inverted. The 5 °C grade has a minimum of 12 h and must also satisfy a substrate temperature 3 °C above the dew point.
- Is an additional zinc-rich primer required on galvanized surfaces?
- Generally no. The zinc layer is already zinc; adding a zinc-rich primer is duplication, and adhesion on a non-smooth galvanized surface is harder to ensure. Epoxy or a manufacturer-specified adhesion primer is more reliable; the specification and coating system table govern.
- How should copper slag abrasive and steel shot/steel grit be compared in quotations to avoid a disadvantage?
- Compare on total cost per m², not tonnage price alone. Copper slag has a low unit price but is single-use, and reserve quantity and disposal must be included; steel shot/grit is recyclable, with costs in dust collection, screening, and replenishment. After conversion, the lower unit-price option is often more expensive per m².
- The work order specifies only two coats of epoxy primer without a model; how can this be contested?
- First clarify whether the owner or the contractor holds the paint selection right, then seek to have the model included in the approved brand list or work order template. This step can only be taken before the framework contract is signed; after signing, only price comparison remains.
- With experience only on 5,000 m³ small tanks, can a bid be submitted for a 30,000 m³ tank contract?
- Qualification review is unlikely to be passed. The experience section of such contracts is screened by tank capacity and area, and the owner sets thresholds based on its own tank sizes; amount and qualifications cannot fill the size gap. First accumulate in-tank experience at the same specification, then bid for this tier.
- In EPC bidding for a coating workshop, at which stage is the paint ultimately specified?
- Not in material inquiries, but in the technical specifications of equipment bidding. EPC packages blasting, dehumidification, and exhaust gas as one system; paint is often written into the process equipment specification table and quoted by equipment suppliers. To gain advance notice, identify which package contains these terms.
- For new workshop configuration, why is asking only about area insufficient?
- Area does not indicate capacity; the number of blasting and painting stations determines the cycle. Also ask how color changes are cleaned, how waste liquid is treated, and which data sheet governs intercoat intervals. The recoat interval is a pair of upper and lower values; missing one will cause problems.
- The tender specifies a brand, but purchasing is the contractor's responsibility; with whom should negotiations be held?
- Approach the bidding construction contractor. The owner fixes the brand in the technical requirements, while the commercial terms include material costs in the package, so purchasing falls to the winning bidder. Your competitors are the other bidders, not the owner; what can be negotiated is the coating system table, delivery schedule, and completeness of quality inspection reports.
- On which platform are shipyard touch-up paint orders of several hundred kilograms published?
- Mostly on each group's own electronic procurement platform, in the separate negotiation and separate signing section, not on the public resource trading network. Such inquiries often allow only three days from publication to deadline; first check whether the material description fixes the model and brand, then decide whether to follow.
- If both brand and model are fixed in the inquiry, is it still worth quoting?
- Yes, but first obtain the buyer's material code; without a code, there is no quotation entry. A fixed model indicates purchasing is proceeding by material number, and the comparison is whether the material number is correct and the delivery date can be met, not who is cheaper.
The three places most likely to go wrong on site
- Taking the easy route from the drawings and omitting the intermediate coat—design specifies 240 µm but only two coats are applied, so neither DFT nor barrier properties meet the requirement.
- Using 'higher zinc content' as the sole criterion for primer selection, ignoring the coating system and application conditions.
- Applying different systems over one another without a small-scale test, leading to large-area detachment after full-scale application.
Not sure how to specify your application? Send us theArea, substrate, corrosive environment, design film thicknessSend us the four items, and we will provide system recommendations and quantity calculation based on the manufacturer's TDS.
Other questions:2. Application and conditions 3. Inspection and acceptance 4. Defects and repair