Paint defects
Dry film failures
Holidays
Description
Smaller areas / spots missed out during application. The applied coat does not hide the previous coat or the substrate, uneven colour/ appearance.
Probable causes
Poor application workmanship and / or the poor application technique
Consequences
Poor appearance, poor corrosion protection
Repair
Apply another coat
Prevention
Improve application technique, frequent WFT measurements.
Orange peel
Description
The surface of the paint film resembles the skin of an orange
Probable causes
Poor levelling of the paint due to high viscosity, fast evaporation of solvent, inadequate atomization. Spray nozzle is too close to surface.
Consequences
Poor appearance, uneven film thickness
Repair
Where aesthetics are of concern, abrade overall, clean and recoat.
Prevention
Proper thinning or heating paint. Use correct thinner. Use correct application techniques.
Pin holes
Description
The formation of minute holes in the wet paint film during application and drying, like holes made by a needle.
Probable causes
Solvent or air trapped within the paint film rising to the paint surface, creating small tubes of air which fail to fill in before the film has set. This is a common problem when coating very porous substrates such as zinc silicates and thermally sprayed metal coatings.
Consequences
Poor appearance, poor corrosion protection, uncertain condition inside the pin hole. Difficult to inspect and repair.
Repair
Abrade, clean overall and apply suitable tie coat as necessary. Simply applying a new coat without any preparation will not solve the problem, the pin holes will re-appear.
Prevention
Ensure proper viscosity, choose thinner with low evaporation rate. Sealer coat or mist coat on porous surfaces. Improve application techniques.
Popping
Description
Small translucent or transparent blisters or bubbles on the coating surface. Often the bubbles burst, leaving a crater which does not fill in.
Probable causes
Air or solvents trapped in the film and prevented to escape due to surface drying of the film, foaming during roller application.
Consequences
Poor appearance, poor protection due to low DFT in the popping
Repair
Abrade, clean and recoat
Prevention
Use sealer coat or mist coat on porous surfaces, proper thinning ratio, correct application technique. Avoid application at high temperatures.
Wrinkle/lifting
Description
Formation of wrinkles on the surface during drying/ curing, often combined with loss of adhesion. Paint removers use this effect.
Probable causes
Paint film is exposed during cleaning or overcoating to solvents which it is not resistant to. Tension created in the surface layer of a film while the underlaying part is still fluid (as in too high alkyd paint film).
Consequences
Poor appearance, useless film.
Repair
Remove defective coating, abrade, clean and recoat
Prevention
Use compatible paints within the system. Use correct wet film thickness. Avoid spillage of strong solvents/thinners. Ensure adequate drying and curing.
Amine sweating, blooming & carbonation
Description
After an amine-cured epoxy coating becomes hard and completely dry, there is a sticky substance on the surface.
Probable causes
Incorrect drying conditions, such as: air temperature and substrate temperature are too low; high relative humidity; poor ventilation; direct heating with gas burners (releasing water vapour and CO2).
Consequences
Very poor adhesion for subsequent coats
Repair
If not to be overcoated and appearance is not important: leave as is. Otherwise use warm water and detergent to remove the amine sweating, test for free amines before overcoating (e.g. Elcometer 139 Amine Blush test)
Prevention
Follow the induction time instructions. Application under correct climatic conditions. Keep correct thinning ratio.
Blushing
Description
After drying the coating has a milky surface with low gloss.
Probable causes
Coating surface is exposed to water/moisture before the coating is dry: Fog, rain or during the drying period, condensation. Fast evaporation of a thinner can cause condensation.
Consequences
Poor appearance
Repair
Abrade, clean and recoat
Prevention
Monitor ambient conditions and adjust application schedule. Dehumidification in enclosed spaces. Correct thinner to be used.
Osmotic blistering
Description
Blisters appearing in the film, normally with liquid inside the blisters.
Probable causes
Soluble salts on the substrate under the film (or retained solvents) attract water from the environment and the resulting osmotic pressure inflate the blisters.
Consequences
Poor appearance, loss of adhesion, loss of corrosion protection
Repair
Remove blistered areas or entire coating system, fresh water wash and repair or fully recoat.
Prevention
Ensure correct and sufficient surface preparation, test substrate for presence of soluble salts (Bresle).
Pin point rust
Description
Small point corrosion observed on the steel surface, like pin (needle) points
Probable causes
Low film thickness; Excessive surface profile; Poor wetting of the substrate; High evaporation rate of solvent/thinner; Dry spray; Poor atomization; Pinhole and/or holiday
Consequences
Poor corrosion protection, poor appearance
Repair
Remove paint and recoat
Prevention
Correct surface preparation, correct application technique, correct film thickness.
Cracking
Description
Paint film is splitting. The splits may be just on surface, through a coat or through the whole coating system down to substrate. The name of this defect varies according to the depth of the cracks and pattern of cracking, e.g. checking, alligatoring, mud cracking, etc.
Probable causes
Cracking is a stress related failure and can be caused by substrate movements, ageing, lack of flexibility in the coating, swelling and drying due to moisture, thick layers consisting of many coats of different hardness (sandwich coats).
Consequences
Poor appearance, lack of corrosion protection
Repair
Remove the cracked coating by grinding and repair the coating system as per specification. For mud cracking of inorganic zinc coatings, sand-blasting to the substrate and re-paint.
Prevention
Avoid too high film thickness. Ensure proper drying / curing and that the coatings in the system are compatible. Alternatively use a more flexible coating system.
Delamination
Description
Film is has lost its adhesion and separates from the surface. The separation may be just the last coat or the whole coating system down to the substrate.
Flaking/delamination of a tank
Probable causes
Poor surface preparation; contamination on the substrate; hard and very glossy substrate, paint defects such as: chalking, flooding, amine sweating; excessive film thickness; exceeded max over coating interval; solvent and/or air entrapment; weathered coating.
Consequences
Poor appearance, poor protection against the environment and corrosion
Repair
Remove the coating with poor adhesion. Prepare the substrate and paint it again as per specification.
Prevention
Proper surface preparation and cleaning before overcoating. Apply subsequent coats before max. overcoating interval is reached.
Avoid high film thickness. Abrade and clean glossy surfaces between coats.
Chalking
Description
A grey / white dusty material appears on the surface after the coating has been exposed to sun light for some time.
Probable causes
Degradation of paint binder (resin) in the coating by ultraviolet light from the sun; weathering; poor mixing
Consequences
Poor appearance, poor adhesion for subsequent coats
Repair
Remove the chalking by cleaning with a detergent and overcoat with a UV resistant coating.
Prevention
Select suitable topcoat for prevailing climatic conditions.
Bleeding
Description
Discolouration of the paint film due to migration of coloured substances from the underlying paint, typically seen when tar or bitumen containing coatings are overcoated.
Probable causes
Coloured substances in under laying coat are re-dissolved by solvents in the new coat. Can also be caused by unsuitable marker pens used by inspectors.
Consequences
Poor appearance
Repair
Remove the whole coating system and apply a non-bleeding coating. Use a paint with flake pigments such as aluminium or glass flake as a sealer coat to reduce the bleeding.
Prevention
Avoid the use of coatings containing tar and bitumen, only use approved marker pens during inspection.
Vacuoles
Description
Voids in the paint film, cross section looks like a Swiss cheese.
Probable causes
Air and/or solvent trapped within the film during drying, often related to accelerated surface drying (skin drying) during high temperature and strong wind/ventilation.
Consequences
Early corrosion, may develop into blisters
Repair
Remove the porous coating and apply a new coat.
Prevention:
Monitor wind and temperature during open air application and control ventilation in confined spaces, to avoid skin drying.
