How to diagnose coating defects: causes of and remedies for blistering, pinholes, cratering and peeling
The key to defect diagnosis is not "identifying what the defect is", butdetermining which layer the cause lies in: substrate preparation, application conditions, mixing ratio, or the material itself. If the cause is misjudged, the repair is wasted work.
Table of contents on this page
- Blistering and yellowing(4 items)
- Pinholes and cratering(3 items)
- Sagging, cracking and peeling(4 items)
- Remediation and responsibility assessment(4 items)
- Others(6 items)
Blistering and yellowing
- What causes coating blistering?
- Four common causes: moisture or condensation on the substrate, salts/oil not removed from the surface, solvent trapped by too short an intercoat interval, and curing in a damp environment. Clear blister fluid indicates a water-borne tendency; yellowed and tacky fluid indicates a solvent or curing agent ratio problem.
- How is coating blistering treated?
- First determine the extent: for local blistering, scrape back to a sound layer, dry, redo surface preparation and then patch coat; for extensive blistering, remove the entire area, because the cause usually lies in the substrate or between coats. Clear blister fluid tending to water-borne is mostly related to substrate moisture or condensation; check the drying conditions before patching.
- What causes blistering with yellowing?
- Mostly incomplete curing or amine curing agent exudation (amine bloom), common under damp, low-temperature conditions. Treatment requires removal of the yellowed layer and recoating after drying. Common under damp, low-temperature conditions; treatment requires removal of the yellowed layer and recoating after drying.
- How is it determined whether a blister contains water or is empty?
- Pierce and observe: water indicates permeation or moisture in the substrate; a dry cavity mostly indicates solvent entrapment or intercoat adhesion failure. The judgement differs and the repair extent differs. Water indicates permeation or substrate moisture; a dry cavity mostly indicates solvent entrapment or intercoat adhesion failure, and the repair extent differs.
Pinholes and cratering
- What causes pinholes?
- Rapid solvent evaporation, excessive film thickness in a single coat, excessive substrate temperature and improper spray distance all allow bubbles to break through the film and form pinholes. Bubbles breaking through the film form pinholes; high substrate temperature and heavy single-coat spraying are common causes.
- What causes cratering and fish eyes?
- Oil, silicone or wax contamination on the substrate or underlying coat causes uneven surface tension, so the film contracts into craters. Silicone contamination is the most difficult to handle and requires complete removal and change of tools. Silicone contamination is the most difficult to handle and requires complete removal and change of tools, otherwise it recurs.
- How are pinholes repaired?
- Sand areas of dense pinholes and recoat; patch scattered individual pinholes separately. For protective systems, a spark test is also required to confirm there is no through-thickness holiday. For protective systems, a spark test is also required to confirm no through-thickness holiday, and retesting is required after repair.
Sagging, cracking and peeling
- What causes sagging?
- Excessive thickness in a single spray pass, over-thinning, too short a spray gun distance or too slow a pass. Film thickness at sagged areas is uneven and usually requires sanding followed by recoating. Film thickness at sagged areas is uneven and usually requires sanding followed by recoating at the specified interval.
- What causes coating cracking?
- Film thickness far exceeding the recommended value, unsuitable intercoat compatibility, or outdoor ageing at the end of service life. Structural cracking must be removed down to a sound layer. Structural cracking must be removed down to a sound layer and cannot be covered by a topcoat.
- What causes paint detachment and delamination?
- Nine out of ten cases lie at the interface: inadequate substrate preparation (rust, oil, salt present), intercoat overrun without sanding, or incompatible systems. A smooth detached surface indicates an adhesion problem; adherent substrate particles indicate a surface preparation problem.
- How is coating whitening treated?
- Mostly poor curing caused by moisture; stop coating and dehumidify before making an assessment. Slight whitening may be sanded and topcoated; patchy whitening or whitening accompanied by a soft film must be removed down to a sound layer and redone, and cannot be covered with a single topcoat.
Remediation and responsibility assessment
- How is local repair carried out?
- Remove the defect down to a sound layer → sand the edges to a feathered transition → clean and dry → patch coat according to the original system (add a tie coat where necessary) → retest film thickness and holiday detection. Remove the defect down to a sound layer, feather the edges by sanding, clean and dry, patch coat according to the original system, and retest film thickness and holiday detection.
- Is re-inspection required after repair?
- Yes. Film thickness is re-measured at repaired areas, holiday detection and adhesion spot checks are performed and recorded in the application record; otherwise later disputes have no basis. Film thickness is re-measured at repaired areas, holiday detection and adhesion spot checks are performed, and the results are recorded in the application record.
- How is responsibility determined when a quality problem occurs?
- First collect evidence: film thickness data, temperature and humidity records, application times, batch numbers and photographs of the defect morphology. Then compare item by item against the application conditions in the TDS; in most cases it can be established whether conditions exceeded limits or the material was abnormal.
- Can a single overall coat be applied directly over the old coating?
- Not recommended. With the cause unresolved, the defect continues to develop beneath the new paint, leading to complete detachment within a few months and higher rework cost. With the cause unresolved, the defect continues to develop beneath the new paint and detaches in large areas within a few months.
Others
- How is surface whitening assessed when an open-air steel structure has stood for a long period before the next coat?
- Whitening is mostly epoxy chalking; overrun is judged by chalking rather than by counting days only; treatment is sanding to profile followed by coating. The recoat maximum at 15°C is 5 weeks; beyond this window, do not apply directly over the surface.
- Can paint be applied directly over white rust on a galvanized layer?
- No. White rust is a zinc corrosion product and the reaction beneath it has not stopped; blistering after direct painting generally spreads outward from the white rust spot. First clean thoroughly with clean water and a stiff brush or light sweep blasting, then sweep again once fully dry before painting; check this layer with priority on long-stored items.
- Bright small particles are found embedded in the steel surface after blasting; is treatment required?
- Yes, they must be removed. These are abrasive fragments embedded between the steel substrate and the coating film; if left, they reduce adhesion and tend to cause blistering and detachment at that point. The IMO coating performance standard explicitly requires removal of abrasive embedded in the coating, and brittle abrasives such as copper slag require inspection of every surface.
- Why do pipe rack supports rust before the pipes?
- Three locations are at a disadvantage: the top surface holds water and dries slowly, paint is twisted off the bolts during installation, and the area cannot be recoated once the pipe presses on the support plate. The common factor is that these are difficult to coat during application and invisible after installation; changing the paint does not solve it.
- Can one coat be omitted by applying the paint thicker?
- No. The recommended DFT is a range; exceeding the upper limit causes sagging, trapped solvent and consequent pinholes, and the manufacturer's warranty does not apply. Rework inside a tank requires erecting and dismantling scaffolding again, ventilation and tank cleaning; this cost is an order of magnitude higher than an additional coat, so the shortcut is more expensive.
- Is a thicker intermediate coat more reliable?
- No. An excessively thick intermediate coat changes properties first when heated and lifts the entire fire protection layer above it. The regulations specifically require control of primer and intermediate coat thickness; increasing thickness plants the root of detachment and is the wrong direction.
The three places most likely to go wrong on site
- Blistering is scraped back locally without checking substrate moisture, and blistering continues elsewhere.
- Cratering is treated as pinholes; the source is silicone contamination but only the paint is patched.
- Coating the entire surface before the cause is determined seals the problem inside.
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 3. Inspection and acceptance