Painting Defects

Blistering

This defect is the appearance of irregular blisters on the paint finish.

This defect can be caused by:

• Not correctly cleaning substrate
• Contamination of the brush, air gun, line etc.
• Using wrong thinner or incorrect amount of thinners
• Old paint surface
• Excess film thickness
• In timber finishes, not allowing the solvent, particularly paint removers , to evaporate before repainting

This defect may be overcome by:

• Cleaning all surfaces free of grease and allowing the solvent to evaporate.
• Using recommended thinner at correct ratio.
• In spray applications, inspect so that water does not build up in the traps, especially in humid weather.
• Check that the new paint is compatible with the old surface.
• Do not apply paint films too quickly and allow solvents to evaporate before re-coating.

Blooming

This defect gives a bloom or white deposit, like the bloom on a grape or plum, after the paint has dried. The cause is the rising of soluble fractions of the pigment rising to the surface on the paint's drying. The remedy for spray paints is to rub the surface down.

Blushing

This is a white deposit appearing on the surface of lacquer films only. The defect is caused by painting with lacquers in high humidity conditions where the water contained in the air condenses on the paint film The remedy is not to paint in humid conditions or to add a strong, active solvent that may stop the blushing.

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Painting Defects

BGAS Painting Inspector Exam Painting Defects Name and Definition

Banding

This defect is found in spray applications where there is heavy application of paint on the outside of the spray pattern with little paint in the canter of the fan.

The cause can be:

• Too much air pressure
• Uneven lapping of the spray gun
• Having the gun too close to the job

The remedy may be found from:

• Use at correct air pressure
• Ensure that the overlap of each stroke is 50% over the previous coat
• Hold gun at the correct distance from the job - about 15 cm for lacquers and 25 cm for enamels. To correct a coat that has been applied, re-coat with double coat using thinner that has been specifically recommended for the paint using indicated solvent ratio and pressure, and ensuring that the gun is held at the correct distance.

Bleeding

This is the migration of the color from a previous coat into the freshly applied top coat. This defect usually occurs when a light color is applied over a dark color, particularly reds and maroons which are prepared by using organic pigments not resistant to solvents or application over a surface contaminated with bitumen where the solvents in the fresh paint dissolve the bitumen.

The remedy is:

• Use a bleed sealer before application of the light color
• Wash the surface with mineral turps if it is contaminated with bitumen.

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BGAS Painting Study Material Chapter 2 continuation

Profile

Surface profile, Anchor pattern, key, Peak to trough height and Amplitude are all expression meaning the cross section of a blasted area, as measured from the top of the peaks to the bottom of the troughs.  The surface profile requirements are given on the specification for the job, e.g. for B. Gas 30 – 75 microns.

Shot blasted profile

Figure 2.1 Terms relating to preparing surfaces

Grit blasted profile 

Figure 2.2 Grit blasted profile

*Hackle – A small surface lamination, which stands upright like a needle after blasting.  Approximately ≤ 13 mm.  Easily removed.

*Lamination (slivers) – Appears to be a longitudinal ‘crack’, one lip curling back, any laminations found must be referred to engineer for ultrasonic check.

continue............

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