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Coating Defects

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Contents

• Definition of Coating Defects

• Types of Coating Defects

• Causes of Coating Defects

• Specific Defects• Surface Preparation• Application• In Service• Inherent• Film Defects

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Flaw that spoils the required appearance and specified performance of a painted surface

Types of Defects

Definition & Types of Coating Defects

• Blistering• Bleeding• Poor Coverage• Loss of Gloss• Loss of Adhesion • Wrinkling

• Spray dust/Overspray• Rash Rusting• Pinholes • Cratering• Chalking• Runs• Orange Peel

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Poor surface preparation• Surface contamination• Surface profile

Causes of Coating Defects

Poor application• Mixing (1-pack & 2-pack)• Use of brush & roller (low dft per coat)• Use of spraying (low or high dft per coat)

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Incorrect Paint Systems

• eg 2-pack over 1-pack systems

• Incorrect system for the environment

• eg Anticorrosive or Antifouling performance needs

Poor Engineering Design

• Inherent design defects (eg stitch welding)

Poor Quality of Paints

• Product/batch related problems

Causes of Coating Defects

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1) Blast Profile Effects

• Profile too small gives poor key for adhesion

• Profile too large means peaks may protrude through the paint film as paint flows away from peaks.

Steel Profile Paint barely covering highpeaks of blast profile.

Low film thickness of paint over steel with large blast profile

Applied Coating

Specific Defects – Surface Preparation

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2) Cissing

• Due to contamination of the surface, (eg where paint can not wet the surface fully) or in the paint.

• The wet paint appears to retract from some areas, and this is often associated with oil or other contamination of the surface

3) Millscale effects

• Detachment & Galvanic Corrosion.

Specific Defects – Surface Preparation

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4) Welding Issues

• Painting over weld flux residues can cause problems as it is hygroscopic (absorbs water and then dissolves in it) and corrosion will break through the coating at these points

• Paint applied over weld spatter will show early breakdown, due to shrinkage stress in the coating.

Specific Defects – Surface Preparation

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1) Blooming

• surface effect if amine & amine-adduct cured epoxies contact with water before full cure.

• unreacted curing agent takes up moisture.

• not detrimental to film performance but must be removed (solvent wiping) before painting.

2) Blushing

• White surface film soon after application at high humidity due to condensation in the film.

Specific Defects – Application

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3) Dry Spray

• To coalesce and form a continuous film, paint must still be fluid when it reaches the surface.

Target steel(panel, ship etc)

SprayGun

Wet Paint mergingfrom spray tip.

Evapouration of solvent fromatomised droplets before they

can reach the surface..

Specific Defects – Application

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4) Lapping

• Brush strokes & spray pass stay visible after drying.

• Most obvious on paints with fast solvents on hot steel - especially if aluminium pigmented.

• Adding slow solvents can help - or end passes at welds.

5) Pin holing

• Circular pits which appear to be free of paint.

• Bubbles break at a stage where the coating has already dried too much for it to flow and repair the broken surface.

Specific Defects – Application

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6) Sagging & Curtaining

Weight of paint on a surface is too much for the flow properties of the paint to support, due to:-

a) Too high thickness applied.

b) Too much thinner added to the paint.

c) Not enough thixotropy for expected wft.

d) Temperature too low / solvent too slow.

Specific Defects – Application

7) Wrinkling

Common on alkyds at high dft, where surface drying of the film occurs while the under portion of the film remains soft for a longer time

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Under theeffect ofgravity.

SteelCoating

Specific Defects – ApplicationSagging & Curtaining

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‘Downward movement of paint on vertical surfaces during drying process.’

Specific Defects – ApplicationSagging & Runs

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‘A rough crinkled surface caused by a drying interference forming a wrinkled skin’

Cause•Applying the paint too thickly•Over coating an insufficiently cured primer•Painting during extreme hot or cool / damp conditions forcing skin to form rather than cure to occur

Remedial Action•Remove wrinkled layers by sanding / scraping•Patch prime and finish allowing adequate cure in between coats

Specific Defects – ApplicationWrinkling

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1) Mechanical Damage

• Scratching etc is an unavoidable fact of life, but certain precautions can be taken in terms of handling times and the coating specified for a particular use.

2) Sulphide Discoloration on Surface

• The reaction of H2S with lead pigments and driers

• May be uniform or spotted (where water droplets have absorbed sulphide from the air)

• Not, generally, a serious problem for long-term performance of the coating, but can be unsightly.

Specific Defects – In Service

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1) Bleeding

• some pigments are solvent soluble

• Tar & Bituminous coatings will bleed through if next coat has strong solvents

2) Chalking

• Common for epoxies, as UV degradation of the resin at the surface causes breakdown and the release of pigment particles.

• It is a surface effect only, and not associated with a true failure. However, surface dust must be washed off before applying further coats.

Specific Defects – Inherent

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UV Light

Long Exposure

UV Light

Most of the coating is not affected

The upper layer is broken down

by UV light

Specific Defects – InherentChalking

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3) Reduced inhibiting effect on weathering

• Primers are formulated to protect steel from corroding, and topcoats are to protect the primers from weathering.

• If primer is exposed, soluble salts will be leached away.

• In red oxide primers, effect might not be obvious, but when they are overcoated, rapid breakdown occurs.

Specific Defects – Inherent

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‘Coating surface has a different appearance from that when it was originally applied.’

Surface Discoloration

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1) Blistering

Caused by:-

• painting over moisture

• painting over soluble salts

• solvent entrapment

• coating permeability to gas

• excessive Cathodic Protection

• painting over porous surfaces

Specific Defects – Film Defects

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‘Formation of gas or fluid-filled voids with the coating.’

Liquid / gas exerts a pressure on coating greater than adhesion strength causing localised detachment. Coating stretches to form hemispherical blister(s). Once internal pressure exceeds coating strength, the coating will rupture leading to holiday in coating.

Specific Defects – Film DefectsBlistering

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2) Bubbling

Caused by:-

• Painting over porous substrates (eg Zinc Silicate) and displacement of air - Mist Coats.

• Wrong application method - some enamels bubble if applied by roller if warm and windy.

• Applying low solids coatings at high dft where surface dries while solvent is still trying to escape, resulting in a mass of tiny bubbles.

Specific Defects – Film Defects

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3) Checking

• Surface cracking which does not penetrate right through the topcoat - and may need a lens to see it.

• Caused by contraction of the topcoat of paint, and common in dark chlorinated rubbers in hot climates.

Specific Defects – Film Defects

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4) Cracking

• Fracture of at least the topcoat of paint with ageing, and occurs when the substrate and coating expand and contract at different rates.

• As paint films age, they become harder and less flexible and so “Something has to give”.

• Thin films tend to crack less than thick films.

Specific Defects – Film Defects

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Steel and coating layers expand and contract at different

rates

Many temperature

cycles

Specific Defects – Film DefectsCracking

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Hairline – fine surface cracks that occur erratically and at random

Checking – As above, but give a uniform pattern

Crazing – Similar to checking, but go deeper and are broader

Cracking – Breakdown penetrates to substrate, corrosion and detachment

Mud Cracks – penetrate to substrate and form uniform pattern. (Zn Silicates)

Cracking Hairline Mud Cracking

Different Types of Cracking

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5) Corrosion (Rash Rusting)

Caused by:-

• low film thickness

• poor surface preparation

• Wrong selection of coating system

Specific Defects – Film Defects

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6) Edge & Corner failure

• Liquids draw away from sharp points.

• Use Stripe Coating to can overcome this.

Specific Defects – Film DefectsLiquid Paint

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7) Peeling, Flaking & Delaminating (Detaching)

• Reporting “there is serious detachment” does not give much information - or help to provide a solution.

• Peeling - when paint curls off a substrate.

• Flaking - when paint detaches in flakes.

• Delaminating - intercoat adhesion failure.

Specific Defects – Film Defects

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Multiple coats representing frequent applications

Note Area of weaknessdue to internal stresses

Specific Defects – Film Defects

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‘Delamination of coatings or entire scheme from substrate’

Specific Defects – Film DefectsDetachment

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Detachment due to surface contamination

Specific Defects – Film DefectsDetachment

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SteelCoat #1Coat #2

Left behind

Flake Front

Flake Back

Left behind

Flake Front

Flake Back

Left behind

Flake Front

Flake Back

Detachment from Steel SubstrateDue to surface contamination, application beyond pot-life etc

DelaminationDue to surface contamination,

coating incompatibility etc

Cohesive failureWeakness within a coat due to

solvent entrapment etc

Specific Defects – Film DefectsCoatings Detachment Inspection

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• Zinc-rich coatings such as epoxy zinc and zinc silicates have quite a number of advantages where top coating is to be performed, as these do not suffer most of the problems described.

• Do not allow subcontractors to amend or adjust any surface preparation or coating system specified, however well-meaning their intentions may be.

• It is the usual practices that have been followed for years that are as wrong now as they were then.

• Always seek professional and reliable advice while preparing coating specifications.

Final Thoughts & Suggestions

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