How to inspect and accept coatings: film thickness, adhesion, pinholes and preparation grades
The two most contentious issues in acceptance are whether DFT is sufficient and whether surface preparation has been achieved. This page sets out the acceptance rules and test methods clearly, so that conclusions can be reached on site by following them.
Table of contents on this page
- Film thickness inspection(5 items)
- Adhesion and integrity(4 items)
- Surface preparation acceptance(4 items)
- Documentation and inspection(2 items)
- Others(9 items)
Film thickness inspection
- What is the standard for coating thickness inspection of steel structure corrosion protection?
- The commonly used '90/10 rule': all readings are not less than 90% of the specified value, and the average of readings below the specified value is not less than the specified value. The number and location of measurement points follow the design documents or relevant standards.
- How is a DFT gauge calibrated?
- Calibrate before each measurement batch with certified foils or a test block of the same material as the substrate; recalibrate when changing substrate, probe, or range. Uncalibrated readings are for reference only and cannot be used as acceptance data.
- Does a DFT gauge measure dry film or wet film?
- Magnetic/eddy-current gauges measure dry film. Wet film is measured with a wet film comb immediately after spraying for process control. Wet film is measured with a wet film comb immediately after spraying for process control; dry film is the acceptance data.
- What should be done if DFT is insufficient?
- Apply additional coating to the specified value, not sanding. Before additional coating, confirm the minimum interval has passed and perform necessary preparation; after additional coating, re-measure points. Before additional coating, confirm the minimum interval has passed and the surface is clean; after additional coating, re-measure points for acceptance.
- How many measurement points are valid?
- Follow the sampling frequency in the standard and design documents (commonly points by area or number of components, with multiple readings averaged at each point). Fewer measurements do not constitute acceptance. Take points by area or number of components and average multiple readings at each point; fewer measurements do not constitute acceptance.
Adhesion and integrity
- How is coating adhesion tested?
- Common methods are the cross-cut test (ISO 2409) and pull-off test (ISO 4624). Use cross-cut for thin coatings; thick-film and heavy-duty systems mostly use pull-off and evaluate the values. Thick-film heavy-duty systems mostly use pull-off and evaluate the values; cross-cut is suitable for thin coatings.
- How are pinholes detected?
- Use a spark holiday detector (wet sponge method or high-voltage spark method), set the voltage according to coating thickness, scan section by section; an alarm indicates a holiday, which requires touch-up. Select the wet sponge method or high-voltage spark method according to coating thickness; an alarm indicates a holiday, and retesting is required after touch-up.
- How are missed areas detected?
- Visual inspection + spark holiday detection. Welds, edges and corners, bolts, and back sides are the most likely missed areas and should be scanned intensively. Welds, edges, corners, and bolt back sides are most likely to be missed; focus scanning on these areas.
- What is checked in visual acceptance?
- Uniform color, no sagging, no pinholes, no wrinkling or cracking, no missed areas; acceptable defect levels are judged according to the contract or standard, not subjectively. Uniform color, no sagging, pinholes, wrinkling, cracking, or missed areas; defect levels are judged according to the contract or standard, not subjectively.
Surface preparation acceptance
- What is the difference between Sa2.5 and Sa3?
- Sa3 is 'white metal': no visible residue on the surface; Sa2.5 is 'near-white': only slight shadows and stain residues are permitted (not more than 5% of area). Heavy-duty systems commonly use Sa2.5.
- How is surface preparation grade accepted?
- Compare with standard photographs (e.g., the Sa2.5 comparator photographs in ISO 8501-1) and measure roughness and cleanliness; roughness generally follows TDS requirements, and both excessive and insufficient roughness affect adhesion.
- Should soluble salts be measured?
- For coastal, chemical, and heavy-duty corrosion protection projects, measurement is recommended (per the ISO 8502 series); excessive salts cause early blistering and rust spots. For inland general atmospheric environments, simplification is permissible per design requirements.
- Can paint be applied directly after rust recurrence?
- No. Rust recurrence requires re-blasting to the specified grade before painting; direct overcoating traps rust under the film, which quickly lifts from within. Direct overcoating traps rust under the film, which quickly lifts from within; re-treatment to the specified grade is required.
Documentation and inspection
- Do steel structure anticorrosive coatings require re-inspection?
- Project acceptance generally requires product certificates, TDS, batch information, and test reports; whether re-inspection is required follows the contract and supervisor requirements. Whether re-inspection is required follows the contract and supervisor requirements; prepare certificates, TDS, and batch information for acceptance submission first.
- What documents must be prepared for acceptance?
- Product certificates and TDS, application records (temperature, humidity, recoat time), DFT records, adhesion/pinhole test records, and third-party test reports where necessary. Application records (temperature, humidity, recoat time), DFT records, and test records are archived by batch; they provide the basis for later disputes.
Others
- After painting in winter, how soon can a pull-off test be performed?
- Wait until curing meets the data sheet conditions; otherwise low readings will be judged as non-conforming. ISO 4624 readings are for reference; the failure location provides the conclusion: failure in the adhesive layer means the test set is invalid, while failure at the interface indicates which layer has the adhesion problem.
- For components such as gates, at which stage is the surface preparation grade accepted?
- At the factory. Gates and curtain wall steel structures are mostly removable components; surface preparation grade, roughness, and first-coat DFT are completed and recorded in the factory, and only topcoat and damaged areas are touched up on site. The contract must specify this hold point; the site cannot certify the factory stage.
- After blasting inside a tank, what dust level is acceptable?
- Assess per ISO 8502-3 by quantity and size; generally the dust quantity must not exceed grade 2 and particle size not exceed grade 3. These figures are usually given in the specification; the standard only defines classification, so consult the specification for acceptance.
- For tank DFT acceptance, should 80-20 or 90-10 be used?
- According to the specification for this contract, both criteria have appeared on different contracts. The ship ballast tank PSPC set cannot be transferred directly to tanks without misjudgment; industrial tanks mostly specify 80-20, and some specify that no point may be below the specified value. Without confirmation of this clause, coat number and material quantity cannot be calculated accurately.
- When fireproofing coating and primer are incompatible, how should this be handled on site?
- Two options. Obtain a certificate from the fireproofing coating manufacturer stating compatibility with the primer specified for this contract; or prepare a test panel on site using the actual production interval and check for blistering, lifting, and adhesion loss, with both parties signing and retaining records.
- How can inorganic zinc silicate primer curing be quickly verified?
- Use the methyl ethyl ketone rub test. Rub on a test panel per ASTM D4752 and rate by grade; the acceptance grade comes from the data sheet or specification, not from the standard itself. Applying topcoat before complete cure tends to cause cracking or large-area detachment.
- Can flange sealing faces be painted?
- No. The sealing face is the ring that takes gasket compression; if paint film is trapped there, preload is first absorbed by the coating, and gasket compression is insufficient. When maintenance requires disassembly, the paint film has hardened and cannot be pried apart. Mask with PTFE sheet or dedicated caps before assembly.
- What voltage should be set for field joint holiday testing?
- Voltage is not a fixed figure; it follows the material: first check the TDS or the applicable standard. The 5 V/µm in GB 50268 Appendix H applies only to polyurethane coatings; for 3PE, heat-shrink sleeves, and epoxy coal tar, another value must be selected. Too low will miss holidays; too high will perforate sound coating.
- What DFT at field joints is acceptable?
- Not less than 80% of the specified thickness for the pipe body. Field joint coating is part of the complete system, not 'a layer that looks painted'. Take readings at multiple points in the field joint area during measurement; the lowest point must also exceed this limit; if insufficient, apply additional coating to acceptance before reporting.
The three places most likely to go wrong on site
- Measuring only the front face of components and omitting back faces, edges, and welds—these are the areas that rust first.
- Failure to calibrate the thickness gauge; readings do not match the acceptance standard.
- Judging surface preparation by eye only, without comparing standard photographs or measuring roughness.
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:1. Selection and systems 2. Application and conditions 4. Defects and repair