A protective coating cannot be judged by looking at it on handover day. A fresh, glossy film can hide a surface that was never properly cleaned, steel that was coated below the dew point, a film that is half the specified thickness or a lining full of pinholes. The defects show up months or years later as rust, blisters and peeling, when fixing them means removing the whole system.
Inspection is how those problems are caught while they are still cheap to correct. This guide explains what is checked at each stage, the common instruments and test methods, and what good records look like. It is general information for owners, project managers, contractors and students. The inspection requirements for a real project come from the specification written by a qualified engineer or coatings specialist, the coating manufacturer’s data sheets and the applicable standards.
Key takeaways
- Ambient conditions: air and surface temperature, relative humidity and dew point are measured and recorded before and during work.
- Surface preparation: cleanliness, profile, soluble salts and dust on steel; profile, soundness, cleanliness and moisture on concrete.
- Film thickness: wet film checks during application and dry film thickness measured by methods suited to the substrate.
- Adhesion and continuity: pull-off tests where specified and holiday testing on linings.
- Hold points and records make sure each step is accepted before the next begins.
Who inspects
On many projects there are two layers:
- Quality control (QC) by the contractor’s own staff, who check their work as it happens.
- Quality assurance (QA) by the owner’s inspector, often an independent third party, who verifies that the work meets the specification.
Hold points are steps written into the specification where work stops until the inspector accepts it. Typical hold points are after surface preparation, after each coat, after repairs and before return to service. Agreeing them at a pre-job meeting, together with test methods, frequencies and how non-conformances are handled, avoids disputes later.
Before work starts
- Review the specification, the manufacturer’s data sheets and the application instructions for each product.
- Confirm that materials on site match the specification, with batch numbers and shelf life checked.
- Check instruments: calibration certificates, and verification against reference shims, standards or test blocks before use.
- Agree the trial area or reference area that will show what acceptable preparation and finish look like.
Ambient conditions
Most coatings have limits for air temperature, surface temperature and relative humidity, and almost all require the surface to be above the dew point. When a surface is at or below the dew point, a thin film of condensation forms. On steel it causes flash rust and poor adhesion; on concrete it adds moisture that some coatings cannot tolerate.
Inspectors measure:
| Reading | Typical instrument | Why it matters |
|---|---|---|
| Air temperature | Thermometer or electronic gauge | Cure rate, viscosity, application limits |
| Surface temperature | Contact surface thermometer | Must be within the data sheet range and above the dew point |
| Relative humidity | Psychrometer or electronic hygrometer | Some coatings react with or are harmed by moisture |
| Dew point | Calculated from air temperature and humidity | Condensation risk |
| Wind | Anemometer, outdoors | Overspray, dry spray, solvent evaporation |
A common rule of thumb is to keep the surface at least 3 °C (5 °F) above the dew point, but the data sheet sets the actual requirement. Readings are taken before work starts and at intervals during the shift, more often when conditions are changing, such as at dusk or when a tank cools overnight.
Surface preparation inspection
Steel
- Cleanliness: compared with the specified grade using the visual references in AMPP (formerly SSPC and NACE) or ISO 8501 standards. Grades describe how much mill scale, rust and old coating may remain.
- Profile: measured with replica tape or a depth micrometer and compared with the range the coating requires. Too low a profile reduces adhesion; too high leaves peaks poorly covered.
- Soluble salts: chlorides and other salts left on blasted steel can draw water through the coating and cause blistering. Surface extraction methods such as the ISO 8502 patch method followed by conductivity measurement are common, with limits set by the specification.
- Dust: checked with a tape test, also described in the ISO 8502 series.
- Compressed air: checked for oil and water contamination with a blotter test before blasting and blowing down.
- Flash rust and contamination after blasting: the surface must be coated before it deteriorates below the specified grade.
Concrete
- Profile: compared with the ICRI CSP chips; see concrete surface preparation and CSP profiles.
- Soundness: pull-off tests on the prepared surface where specified.
- Cleanliness: no laitance, curing compounds, oils or dust.
- Moisture: measured by the method and limit the coating manufacturer specifies.
- Repairs and cracks: completed and treated as the specification requires.
Mixing and application
Multi-component coatings must be mixed in the correct ratio, for the correct time, and used within their pot life. Inspectors check:
- Batch numbers and expiry dates.
- Mixing ratio and method, induction time where required, and the amount and type of any thinner, which must be within the data sheet limits.
- Equipment set-up: spray tip sizes, pressures and, for plural-component rigs, temperatures and pressures on both sides. Our guide what is polyurea describes this for fast-set elastomers.
- Stripe coating of edges, welds, bolts and corners where specified.
- Recoat windows: the minimum and maximum times between coats at the actual temperature.
Wet film thickness
During application, the applicator and inspector use a wet film comb or wheel gauge to check that enough material is going on. Wet film thickness relates to dry film thickness through the volume solids of the coating: as an approximation, dry film thickness equals wet film thickness multiplied by the volume solids fraction, adjusted for any thinner added. Wet readings are a control tool, not an acceptance measurement. They are less useful for very fast-setting materials, where material consumption and dry readings are relied on instead.
Dry film thickness
On steel
Dry film thickness (DFT) on steel is measured with magnetic or electromagnetic gauges, and on non-ferrous metals with eddy current gauges. AMPP and ISO publish procedures, including the SSPC-PA 2 procedure and ISO 19840, that cover gauge verification, adjustment for surface profile, the number of readings per area and how to judge acceptance. ASTM also publishes a method for non-destructive DFT measurement on metal substrates. The specification states the minimum and maximum DFT and which procedure applies.
Both limits matter. Low DFT reduces barrier protection and increases the risk of pinholes. Excessive DFT can cause cracking, solvent entrapment, poor cure and sometimes falls outside the conditions of a certification listing, as noted in coatings for drinking water tanks.
On concrete
Magnetic gauges do not work on concrete. Options include:
- Ultrasonic gauges that measure coating thickness on non-metallic substrates, where the coating and concrete give a readable interface.
- Destructive methods, cutting a small groove or taking a small coating sample and measuring it under magnification, followed by repair.
- Test plates or panels coated alongside the work and measured with conventional gauges.
- Material consumption compared with the area covered, accounting for profile and losses.
Because profile on concrete varies, the specification should state how thickness is measured and what counts as a pass.
Cure
A coating must be cured before it is put into service, recoated after its maximum window, or tested by some methods. Checks include the time and temperature history against the data sheet, hardness tests, and for some coatings a solvent rub test described in ASTM and other methods. Amine blush, a waxy film that some epoxies form in cold, humid conditions, must be detected and removed before recoating.
Adhesion
Adhesion tests are often destructive and are done at agreed locations or on test patches:
| Method | What it measures | Typical use |
|---|---|---|
| Pull-off test | Force needed to pull a bonded dolly off the coating, and where the failure occurs | Steel (ASTM D4541, ISO 4624) and concrete (ASTM D7234) |
| Knife test and cross-cut or X-cut tape test | Qualitative adhesion of thin films | Mainly thinner coatings on steel |
Pull-off results are recorded with the failure mode: in the substrate, at the bond line, between coats or in the adhesive. A low value with failure in weak concrete tells a different story from the same value with failure between coats. Test areas are repaired afterwards.
Holiday testing
Linings for immersion, buried service and tanks are usually tested for continuity with low-voltage wet sponge or high-voltage spark testers. Thickness is measured first so the voltage can be set correctly. See holiday testing explained for how this works, and wastewater tank and wet well linings for an application where it is routinely specified.
Visual inspection of the finished film
- Runs, sags and curtains.
- Dry spray and overspray.
- Pinholes, bubbles and craters.
- Missed areas, thin edges and shadowed areas behind obstructions.
- Contamination such as dust, abrasive or insects in the film.
- Color and gloss consistency where appearance matters.
Defects are marked, repaired according to the manufacturer’s procedure and re-inspected.
Records
Good records typically include:
- Daily ambient readings with times and locations.
- Surface preparation results: cleanliness grade, profile, salt tests, moisture.
- Materials used: products, batch numbers, quantities, thinner.
- Wet and dry film thickness readings referenced to locations.
- Adhesion and holiday test results, with instrument settings.
- Non-conformance reports and how each was resolved.
- Photographs.
- Instrument serial numbers and calibration or verification records.
These records support warranty claims, help diagnose any future failure and show what maintenance is due. Our guide to why coatings fail describes the common failure types and what an investigation looks at.
Inspector qualifications
Coating inspection is a skill that improves with training and experience. Recognized certification programs exist, including those run by AMPP and by FROSIO, and ICRI offers programs related to concrete surface repair. Many owners require inspectors to hold a specific certification level. Certification shows training, but the inspector also needs to know the specific products, substrates and standards on the project.
Frequently asked questions
Can the contractor inspect its own work? Contractor QC is normal and valuable, but owners on critical projects usually add independent QA.
Is a visual check enough for small jobs? For decorative work it may be. For protective coatings and linings, at least ambient conditions, preparation and thickness should be checked and recorded.
Who decides if a reading is acceptable? The specification and data sheet. Where they are silent, the inspector should raise it rather than make up a criterion.
Bottom line
Coating inspection follows the work from start to finish: conditions and dew point, surface preparation, mixing and application, wet and dry film thickness, cure, adhesion and holiday testing, with hold points between stages and records throughout. Each check uses verified instruments and methods named in the specification. Done well, it turns a coating from an assumption into documented protection. The acceptance criteria themselves come from qualified engineers, the manufacturer’s data sheets and the applicable standards.