Coating defects atlas

40 defects every coating inspector must recognise, each drawn as you see it and in cross-section, with the causes, how to prevent it, how it is rated and how it is repaired.

BlisteringRaised domes in the film, from pinhead size to several centimetres, filled with liquid or gas.Rust-through (rusting)Rust spots, rust staining or general rusting showing through the coating, often first at edges, welds, bolts and damage.Pinpoint rustingMany tiny rust spots spread over the surface, often soon after coating.Undercutting (underfilm corrosion)Corrosion that spreads under the coating from a scratch, edge or holiday, lifting the film as the rust grows.CrackingBreaks in the film, from fine hairline cracks in the top coat to deep cracks down to the substrate.Mud crackingA network of cracks that splits the film into plates, like dried mud.Flaking and delaminationPieces of coating lifting and falling away, either a top coat from the coat below (intercoat) or the whole system from the substrate.ChalkingA powdery layer on the surface that wipes off on a finger or cloth, with loss of gloss and colour.PinholesTiny holes through the film, often in groups, that can reach the substrate.Runs and sagsDrips, curtains or teardrop shapes on vertical surfaces where the wet paint has flowed downward.Orange peelA dimpled surface like the skin of an orange.Dry sprayA rough, sandy, matt surface.Amine blushA greasy, waxy or hazy film on the surface of an amine-cured epoxy, sometimes with white staining.Cratering (fish eyes)Small round holes or dimples where the wet paint has pulled away from a spot, often with a raised rim.Solvent entrapment and poppingBubbles, blisters or small craters in the film, and a film that stays soft.WrinklingA rough, shrivelled surface of ridges and valleys, like the skin of a prune, often on flat surfaces where a heavy coat was laid.Alligatoring (crocodiling)Wide, deep cracks that divide the top coat into islands, like the hide of an alligator.CheckingFine, short breaks in the surface of the top coat that do not go through it.Delamination (intercoat adhesion failure)A coat parts from the coat below in sheets or patches.PeelingThe film lifts in larger, flexible sheets or strips, often from edges, seams or penetrations.LiftingShrivelling, swelling or puckering of a coat soon after the next coat goes on, often in a ring round the edge of a repair.Blushing (solvent blush)A milky, cloudy haze or pale, low-gloss spots on a drying film.BleedingStains or a colour change in a new top coat, as patches, halos or an overall shift, where colour moved up from below.Crawling and cissingThe wet paint pulls back from spots on the surface and leaves bare or thin areas, sometimes with a speck of contamination in the middle.OversprayA rough, sandy or powdery layer of dry spray particles on nearby steel, at the ends of spray passes and on adjacent members.Holidays and missesBare or thinly coated spots: large misses are plain to see, while pinholes and small holidays often are not.Cathodic disbondmentThe coating loses adhesion and lifts, as blisters or larger areas, around a holiday on a buried or submerged structure under cathodic protection.Filiform corrosionThin, thread-like tracks under a thin film, spreading from a scratch or cut edge.Flash rustingA thin yellow-brown to red-brown film of rust that forms on bare steel within minutes to hours after waterjetting or wet blasting.Zinc salts (white rust)White, grey or black powdery deposits on galvanizing or on zinc primer, worst where items were stacked wet.Edge corrosion (thin film at edges)Rust spots or streaks along cut edges, flanges and corners while the flat faces are still sound.OutgassingBubbles, craters and pinholes that appear minutes to hours after coating concrete or a porous zinc primer.Contamination and inclusionsLumps and rough, peppery spots in the film from dust, spent abrasive, paint skins, fibres or insects.Fading and discolorationA paler, washed-out or yellowed surface, often with a loss of gloss.UndercureA film that stays soft or tacky, dents under a thumbnail, or looks normal but fails in service.Brush marksLines of ridges and furrows that follow the brush; very coarse ones are called ropiness.EfflorescenceWhite, crusty or fluffy salt deposits on concrete or masonry, often in bands along mortar joints.Bugholes (surface voids in concrete)Small pits in formed concrete, usually up to about 15 mm across, many hidden under a thin crust of mortar.LaitanceA thin, weak, powdery layer of cement and fines on the top of concrete, sometimes with fine crazing.Steel surface imperfectionsWeld spatter, rough welds, slivers, laminations and sharp edges.

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

Also: Concepts · Standards · Defects atlas · Glossary · Calculators · Guides.