Tag: Paint Film Defects

Paint film defects can appear during or immediately after application or become more apparent after the coating is cured. While there is no standard convention for the nomenclature of film defects, this article will separate film defects into the two categories mentioned above.

Paint film defect causes

The largest number of paint defects is from dirt particles¹embedded in the paint. Most other paint defects are the results of:

• lack of cleanliness
• surface preparation
• application error
• attention to detail

Surface tension

Many coating defects are related to surface tension issues. Surface tension is the elastic tendency of liquids that make them acquire the least surface area possible. This occurs when the forces at the interface of a liquid differ from those within the liquid, attributed to uneven force distribution of molecules at the surface. A common unit of surface tension is dynes/cm² (force/unit area).

For example, applying a coating with a higher surface tension than the substrate may cause dewetting, crawling, pinholing, holidays and telegraphing.

Likewise, the difference in surface tension at the paint surface can result in cratering or fisheyes.

Table 1: Surface tension of paint Solvents

Solvent	Surface Tension Dynes/cm
Water	72.8
Toluene	28.4
Isopropanol	23.0
n-Butanol	24.8
Acetone	25.2
Methyl propyl ketone	26.6
Methyl amyl ketone	26.1
PM acetate	28.5

 

Table 2: Liquid surface tension of Polymers used to reduce surface defects

Polymer	mj/m2
Poly(dimethylsiloxane)	22.6
Poly nButyl Acrylate	33.7
Poly nButyl Methacrylate	31.2

 

Highly polar molecules (e.g. water) have a higher surface tension than less polar materials (see Tables 1 and 2). Surface defects can often be reduced or eliminated by using small amounts of additives with low surface tension such as polydimethyl siloxanes (DMS), poly butyl acrylate or poly 2-ethyl hexyl acrylate. These additives tend to migrate to the surface to help flow and leveling.

Table 3: Defects that can occur during or soon after application

Defect	Appearance	Causes	Remedy
Crawling	Uneven film thickness, dewetting	High surface tension paints applied to a substrate with lower surface tension. For example, paint on steel with oil on the surface	Proper surface cleaning of metallic or plastic surfaces
Craters/fish eyes	Small round depressions in the surface of the coating	Small particles of a low surface tension contaminant (e.g. oil, grease, silicone oil, wax) on the substrate or that embeds in the coating	Proper spray booth air filtration and the contaminant elimination. The addition of surface wetting agents such as DMS and/or polyacrylates with a low glass transition (Tg).
Crazing, cracking	Small cracks formed in the coating. This can occur on recoat or if coating is applied to solvent sensitive plastics	Application of coatings on plastics where the paint contains strong solvent that solvates the underlying coating layer or plastic substrate	Use solvent that will not crack or craze the plastic. Test spot resistance of substrate with suitable solvent.
Dirt, contamination	Small raised imperfections in the surface of the coating	Surface not carefully cleaned. Dirty spray booth and/or booth filters. Pressure in the spray booth too low. Unsuitable work clothes. Inadequate paint filtration	Ensure cleanliness of the environment where the coatings are applied
Loss of gloss, blush	Areas of low gloss or a white haze	Humidity condenses on the wet paint due to the cooling effect of solvent evaporation when the substrate temperature is below the dew point. Causes: Unsuitable reducers Poor air circulation in drying oven Film thickness too high or low	Proper humidity control
Mottling	Uneven appearance of metallic paints	Dirty spray gun nozzle Incorrect air pressure Incorrect reducer Faulty spray technique Incorrect spray viscosity	Use proper viscosity cup to obtain spray viscosity. Clean and maintain spray guns on a regular basis. During application maintain spray gun parallel to the substrate and maintain correct distance from gun to substrate. Follow Technical Data Sheets instructions.
Poor hiding	· Uneven paint coverage	Nonuniform substrate surface Uneven or inadequate paint coverage to mask the substrate color	Uniform and sufficient paint application to obtain proper hiding.
Runs and sags	Drips and sags	Paint applied too thick or too wet to a vertical surface and the force of gravity overcomes the forces resisting the downward flow of paint (viscosity). Temperature too low to enable proper solvent evaporation (solvent born paint), or humidity too high (waterborne paint).	Adjust low shear viscosity of paint with appropriate thickener. Use proper reducer and viscosity adjustment for environmental conditions. Adjust spray gun and apply thinner wet coats. If a waterborne paint, apply paint in a lower humidity environment.
Skips/holidays	Incomplete paint coverage	Paint applied too thin Minute areas on the substrate surface of low surface tension, causing inadequate film flow and coverage.	Proper paint application and ensure surface cleanliness.
Striping, banding	Stripes of uneven paint appearance (e.g. differing color)	Uneven paint application	Use proper viscosity cup to obtain spray viscosity. Clean and maintain spray guns on a regular basis. During application, maintain spray gun parallel at the correct distance to the substrate and maintain
Telegraphing	Highlighting of the surface of the coated substrate through the coating. Such defects as fingerprints, sand scratches and water spots on the substrate become visible on the coating surface	Coating with high surface tension applied to a substrate with lower surface tension. e.g. Fingerprints or silicone oil on a substrate surface.	Ensure that the substrate is thoroughly clean and absent of low surface tension oils and fingerprints.
Wrinkling, lifting, aligatoring	Upon applying an overcoat, the existing paint film shrivels, wrinkles or swells; may also occur during drying.	Solvents in the new paint swell the underlying paint finish.	Allow sufficient cure times of underlying paint Ensure that the new paint is compatible with the undercoat Proper application of the new paint (not too wet).

 

Table 4: Defects that are more apparent after cure

Defect	Appearance	Causes	Remedy
Air entrapment	Similar to solvent popping or bubbles	Paint pump sucking air when paint level is low. In two component urethanes, moisture present reacts with isocyanate to cause CO2 generation.	Proper attention to paint line conditions. Ensure use of urethane grade solvents and proper spray gun air filtration through desicant. Addition of moisture scavenger in paint.
Barnard Cells	Hexagonal pattern in the surface of a cured paint film.	Convection pattern from pigment segregation as a result of surface tension differentials	Adjust formulation to overcome flooding and differential surface tension at surface
Blisters	Bubbles near the surface of a film during oven cure that do not break through the surface.	Viscosity of the surface of the film increases to a high level, trapping the volatile solvent at a lower level.	Proper oven staging to enable slow release of solvent. In an acid catalyzed system, use an acid salt to slow the cure and enable solvent release. Increase flash time before bake. Use slower evaporating solvent. For spray application, apply additional thinner coats to build film rather than fewer thick coats. For waterborne coatings, use a dehydration bake lower than the boiling point of water, followed by a second bake to cure.
Orange peel	Rough surface that resembles the surface profile of an orange	Paint applied at high viscosity or under conditions deleterious to proper flow and leveling.	Adjust paint to proper viscosity with correct reducer per technical data sheets. Apply at proper fluid delivery rate and atomizing air pressure.
Solvent pop	Broken bubbles at the surface of a film that do not flow out during oven cure	Viscosity of the surface of the film increases to a high level, trapping the volatile solvent at a lower level. The bubbles break the surface when the solvent volatilizes.	Proper oven staging to enable slow release of solvent. In an acid catalyzed system, use an acid salt to slow the cure and enable solvent release. Increase flash time before bake. Use slower evaporating solvent. For spray application, apply additional thinner coats to build film rather than fewer thick coats. For waterborne coatings, use a dehydration bake lower than the boiling point of water followed by a second higher bake to cure. Lastly, the use of lower Tg resins along with lower dry film thickness decrease popping.

 

Search Prospector for formulating remedies to overcome paint film defects:

Defect	Remedy material
Crawling and substrate wetting	polyether modified polydimethylsiloxane (PDMS) nonionic surfactant for waterborne paints
Craters and fish eyes	PDMS polyalkyl acrylates
Runs and sags	Viscosity modifier to increase low shear paint viscosity
Telegraphing	polyether modified polymethylalkylsiloxane to increase low shear viscosity (solvent borne and waterborne)
Air entrapment	surface tension depressant low molecular weight acrylic defoamer (waterborne)
Solvent pop, blisters	For melamine cure systems: blocked acid catalyst polyalkyl acrylate slow evaporating solvent

 

To read more, please click here to head over to UL Prospector.

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Ron Lewarchik, Author of article & President of Chemical Dynamics

As a contributing writer, Ron pens articles on topics relevant to formulators in the coatings industry. He also serves as a consultant for the Prospector materials search engine, advising on issues related to optimization and organization materials within the database.