PAINTING 101 : Paint Defects

Session 5: Paint Defects

 Research has revealed that over 80% of paint failures in developed nations are purely due to poor surface preparation and only about 10% due to product failure. Poor paint performance is attributable to a number of factors but often in reality end users apportion the blame to the product itself without looking at other factors.

These defects may occur during paint storage, during application and drying/curing or during the service life of the dry coating. This topic aims to give a brief of these defects and how best we can avoid or remedy them.

 Common defects during paint storage: 

 i. Skinning

•  Characteristics: Formation of an insoluble skin like covering on surface of liquid paints and varnishes exposed to air.
• Possible causes – container lid not air-tight, poor formulation (not enough anti-skinning agent or too much surface drier) and very hot storage conditions
• Prevention: Use airtight container, avoid unnecessary opening of tin, the addition of slow evaporating solvent and good storage conditions
• Remedy: Carefully remove skin and stir the paint till homogeneous. If any bits are present after stirring, sieve paint through a clean fine cloth before use. 

ii. Settling

• Characteristics: Settlement of solid particles to the bottom of the tin. The soft settlement is easily mixed by just stirring while the hard settlement is not easily mixed and requires specialised machinery.
• Possible causes: insufficient stirring during storage, storing paints under hot conditions or for too long (often beyond expiry date) and use of inappropriate thinner.
• Prevention: Use recommended thinner, avoid long storage and ensure good storage conditions.
• Remedy: Stir in soft settlement till homogeneous. If hard settlement, return to the manufacturer for mixing using appropriate machinery. 

iii. Gelling

• Characteristics: Jelly-like flow of paint caused by a bacterial attack of binder or thickener or both
• Possible causes  Contaminated tools or thinner, mixing of different brands or different paint types
• Prevention: Use clean tools/ solvents and avoid mixing different brands or paints
•  Remedy: Discard the paint. 

iv. Fouling

• Characteristics: Bad smell due to breakdown in resins, thickener and biocides
• Possible causes  Poor formulation (no or inadequate biocide), expired paint or contamination and poor storage conditions
• Prevention: Good storage conditions and disinfect the warehouse/ factory
• Remedy: Discard the paint to avoid contamination of good paint.

Common defects during application. 

i. Sagging, Running or Curtaining

• Characteristics: Paint or varnish flowing or drooping like a draped curtain of material from upper vertical surface to the lower part forming tear-like or wavy appearance.
• Possible causes: Too much thinning or wrong/ slow thinner. Paint coat too thick or insufficient dry time between coats. Building, paint or surface too cold. 
• Painting is done on a glossy surface without adequate surface preparation i.e., abrading.
• Prevention: Use correct thinner. Apply thin, even coats. Avoid application in cold conditions. Good surface preparation sand & clean
• Remedy:  Sand down to a smooth surface and repaint. 

ii. Cissing

• Characteristics: Paint fails to form a continuous film on the surface, peeling back when it is applied leaving small round bare patches.
• Possible causes  Greasy or oily surface, application of emulsion paint over oil paint and painting very smooth and shiny surfaces.
• Prevention: Ensure the surface is free of oil and grease. Sand shiny surfaces before painting. Apply an undercoat or sealer before overcoating oil paint with emulsion paint.
• Remedy: Washed off the wet coat immediately with Crown White Spirit. Then thoroughly clean the surface and allow to dry before repainting. If the coating has been allowed to harden, sand down to even film and repaint.

iii. Bittiness

• Appearance: Pieces of dirt, grit or skin in the paint film
•  Causes: Use of dirty brushes or rollers, a dirty surface, bits of dried-up paint that gets stirred in (from removing skin) or pollution from the atmosphere.
• Prevention: Use clean tools. Strain paint through a cloth before use and clean surface before painting.
• Remedy: Rub down to a smooth surface and re-coat. 

Common defects during drying/ curing

i. Wrinkling

• Appearance: Undulation, wrinkle like film
• Cause: Too thick coats. Painting over insufficiently dry films. Poor formulation – too much surface drier.
• Prevention: Apply thin coats. Observe overcoating intervals.
• Remedy: Remove film, sand, clean and repaint.

ii. Bleeding

• Characteristics: A change in colour of the paint that results from the pigment, or other colored material in the surface beneath the coating, moving upward into the applied film and becoming visible.
• Possible causes Soluble dyes or pigments (especially red) on the existing coating. Strong solvent in topcoat attacking underlying coat. Frequent brushing on the same spot
• Prevention: Thoroughly clean the areas to be painted before and after sanding, especially when applying lighter colours over darker colours. Avoid using lighter colours over older shades of red without applying a sealer first. Select the right thinner and use correct paint system.
• Remedy:  Allow to dry thoroughly, sand and seal with +3Masonary or Penetrating Primer before repainting. 

iii. Pinholes, popping or bubbling

• Appearance: Small pinholes on the paint film
• Cause: Moisture in the substrate. Fast-drying thinner. Too thick coats
•  Prevention: Avoid painting in direct sunlight. Allow undercoats/ primers to dry adequately before overcoating. Use appropriate solvent and apply appropriate film thickness
• Remedy: Sand out the bubbles and repaint

iv. Slow Drying

• Characteristics: Paints taking longer than specified to dry
• Cause: High humidity or low temperature during painting. Poor ventilation or moist surface. Oily, greasy or alkaline surface. Wrong or slow drying solvent
• Prevention: Ensure good ambient conditions. Good surface preparation and use of correct solvent
• Remedy: Remove coat and repaint

Common defects on dry film 

i. Algae/ Fungus

Appearance: Greenish or black patches which disappear when bleached.
 Cause: Damp areas receiving little or no sunlight. Painting over an area that has had mould. Reusing opened containers that have received contamination from application transfer.
 Prevention: Good surface preparation – use Fungicidal Wash or bleach to sterilise walls before painting. Avoid using fouling paints.
Remedy: Apply a few drops of house hold bleach, if the discoloured areas disappear, it’s most likely mould. Remove the source of moisture by fixing the leak source, sealing or ventilation. Apply Fungicidal Wash, scrub and rinse thoroughly. Recoat with paint that is protected with dry film biocide.

ii. Chalking

Appearance: Formation of powder consisting of broken down binder and freed pigment particles on the surface of coating exposed to temperature, UV and moisture. Chalking identified by rubbing the surface with a damp cloth which results in a light deposit on the cloth and the restoration of the colour to the cleaned surface.
 Cause: Using interior quality paint for exterior work. Using low-grade paint with a low binder and high pigment content.
 Prevention: Good surface preparation – use Fungicidal Wash or bleach to sterilise walls before painting. Avoid using fouling paints.
Remedy: Remove the friable chalk and rinse with water. Check for any remaining chalk after the surface has thoroughly dried. Repeat the cleaning process if the powder is still present. Apply 1 coat alkali-resisting primer and recoat with the Premium quality coat. 

iii. Poor Opacity

Appearance: Underlying surface still partially visible after being painted over.
 Causes: Application of over-thinned paint. An insufficient number of coats. Wide colour changes e.g., from black to white with insufficient coats. Paint applied without stirring. Over spreading of paint. Application over porous surfaces without adequate surface preparation.
 Prevention:  Use Crown Universal Undercoat especially when making wide colour changes. Stir paint before use. Avoid over-thinning paint.
Remedy: Apply extra coats until the surface is obliterated. 

URL LINK : Painting 101 Paint Defects