Coating Failure
   

Coating Failure
 
Occurs when the coating no longer provides the service or protection for which it was intended.
   

Causes
  • Improper Coating selection
  • Design and manufacture of work piece
  • Surface preparation
  • Application
  • Inspection
  • Maintenance


Types of Coating Failure

 

APPLICATION RELATED

Improper thickness
 
If coating is too thin, pinpoint corrosion can occur. If too thick, coating can check, alligator or crack. A minimum and maximum DFT should be specified for each coat. Coating should be applied in an even, uniform film, allowing coating to wet the surface.

A high film thickness can have more problems than a low film thickness because of development of stress during the coatings drying and curing cycle and is more difficult to correct than low DFT. Overly thick coatings can retain solvent, leading to problems with cure and possibly lead to coating failure.


Holidays / Discontinuities
 

Inattentive application may result in bare spots, misses or even thin spots in a coating leaving the substrate uncoated, or insufficient thickness to be of use. In this situation pinpoint corrosion and premature coating failure is likely to occur.

Coatings should be applied in a smooth uniform film, the application must ensure all areas are coated, even hard to reach areas, and should overlap each spray pass 50%.


Pinholes (Pinholes may be defined as holidays)
 
Are small visible holes in a coating due to application of a thicker organic coating over a porous surface (eg. Coating epoxy over concrete or over inorganic zinc) or holding the spray gun too close to the surface, which can force bubbles into the coating. Coating with wrong solvent balance at high ambient temperature.


Overspray
 
A rough surface, which looks like abrasive dust on the surface of the coating. Overspray is poorly bonded to the painted surface – leading to adhesion failure of subsequent coat if not removed.

Ensure that applicator is not arching the gun. Distance is proper and optimum between gun and surface. Apply coating in thin multiple passes overlapping 50%. Coating is fluid enough to wet surface.

Cratering ( Fisheyes)
 
  • Localized thin areas look like craters distributed randomly over the surface, caused by oil on the surface or by oil in the spray atomizing air. Ensure absence of oil and check the surface for contamination.
  • Improperly formulated coating material.
  • If fisheyes occur, roughen the cratered surface and apply another coat by brush, working coating into the cratered areas.



Lapping
  When each stroke or spray gun is visible after the paint has dried.


Blushing
  Appearance of a white film, or a loss of gloss on painted surface after application.
Caused by moisture on paint films which, have been applied during period of excessive humidity.


Sagging
 

The effect of continuous “runs” on vertical surfaces, generally caused through one or more of the following:

  • Excessive paint per coat
  • Excessive thinner
  • Insufficient paint body
  • Temperature too low
  • Failure to completely mix in pigments or converter


Mud Cracking
 
Occurs when highly filled coatings, particularly zinc rich coatings, are applied too heavily. Occurs if solvent or water evaporates rapidly.


Pinpoint Rusting (Or Flash Rusting)
 
  • Flash rusting occurs when an inadequate thickness of primer coating has been applied, and left to withstand the weathering process too long.
  • Flash rusting occurs when a blasted surface is exposed to the environment for a long period of time, or if blasting takes place during high humidity.
  • This can occur with zinc coatings that have too low zinc loading, or with some zincs to which pigments are added.
  • If peaks of blasted steal are high and are not completely covered with primer.

 

Failure to Cure
 

The failure of a two component product to cure (dry) will be the result of one or more of the following:

  • Wrong / incompatible thinner
  • Dirty / contaminated equipment
  • Failure to add converter
  • Excessive thick application
  • Application at too low a temperature

 



ADHESION RELATED
 

Adhesion related failure can often be related to application or pre-application situations:

  • Poor surface preparation
  • Presence of soluble salts
  • Contamination of intermediate coat
  • Allowing chemically cured coating to fully cure prior to overcoaitng.


BLISTERING
 

Caused by gases or liquids within or underneath the paint, which exerts pressure stronger than the adhesion of the paint at the area under stress, and which will usually be related to one or more of the following:

  • Soluble pigments in the primer
  • Soluble chemical salts
  • Non impervious coating used for immersion service
  • Other contaminants – such as oils, waxes or dirt on the substrate
  • Incompatibility – if an unsuitable primer is used and later over-coated with a high performance coating, the system may not have the necessary adhesion or physical properties to provide adhesion for the high performance coating.
  • Other coatings – The second coating may not soften the first coating sufficiently to create a strong bond, therefore indicating a degree of incompatibility, and the possibility of future delamination
  • Retained solvent – when solvent is not released from the coating prior to overcoating.
  • Cathodic – Due to high or excess voltage.


Intercoat Delamination
 
  • Loss of adhesion between coats in multicoat system. Most common where repair or maintenance coatings are applied over cured coatings.
  • Application of a coating over another coating that has fully cured.

Undercutting
  Action of rust under the coating usually formed around small breaks in the coating.
Use of inhibitive pigments in the primer is intended to prevent the spread of rust beneath apparently sound coatings.


Checking / Alligatoring / Cracking
  • Checking – Slight breaks in the film that do not penetrate to the substrate.
  • Alligatoring – Breaks which are wide and extensive, but do not penetrate to the substrate.
  • Cracking – Breaks extending to the substrate.
Lifting
  Softening and expansion of coating by solvents of a newly applied paint.


Wrinkling
  Formation of furrows and ridges at the surface of a paint system.


Chalking
 
When the surface of the coating turns to powder due to degradation of a binder over a period of exposure to weather, gradually degreasing coating thickness to a point where it ceases to protect the surface.


Abrasion Damage
  Physical damage caused by impact or rubbing movement.