Paint Defects 2 – Pinhole

The formation of minute holes in the wet paint film during application and drying, due to air or gas bubbles, which burst, giving rise to small craters or holes which fail to coalesce before the film dries.

Solvent or air entrapment within a paint film. A common problem when coating porous substrate such as zinc filled primers, zinc silicates and metal spraye coatings etc. Pinholes can also be caused by incorrect spray application or incorrect solvent blend.

Pinholing is a common type of application failure. It may result from several causes. The formulation of the coating itself can cause pinholes, primarily because of improper solvent balance when solvents evoporate too rapidly at one stage of the drying process. Another, more common cause is Improper application, usually during spraying. The spray gun may be held too close to the surface with excessive atomization pressure , or excessive material pressure may be combined with low atomization pressure.

A third cause of pinholes may be the surface itself. Concrete may already contain innumerable bugholes that must be filled if an impervious coating is to be obtained. Pictorial descriptions of bugholes in concrete are found in the ASTM Manual of Coating Work for Light Water Nuclear Power Plant Primary Containment and Other Safety-Related Facilities.

One cause of pinholing is the top coating of inorganic zinc primers with organic top coats. During a period shortly after the inorganic zinc coating has been applied, it remains a porous film and solvents form the organic top coats can easily penetrate into the inorganic coating. When the top coat is applied in the sun or under warm conditions, the penetrated solvent evaporates rapidly causing vapor pressure within the inorganic zinc and under the organic top coat. This vapor pressure may create small blisters or bubbles which, when they break, cause pinholes to form. Top coats with slow drying characteristics or high solids and a low solvent with content help to alleviate this condition.

Pinholes are an immediate problem. Once they occur, they will persist no matter how many subsequent coats are applied. As one coat is sprayed over another, or over pinholes in the substrate, the existing pinholes will act as a reservoir for solvent vapor from the following coat. The vapor pressure in the pinholes will then couse a bubble in the following coat that will eventually break, leaving a passage to the original pinhole and the underlying surface.

Mechanical force is necessary to fill the pinholes with liquid coating. This filling is accomplished by brushing  a coat into the pinholed area. Several passes over the same area may be required to fill all pinholes.

Pinholing occurs most readily in lacquers and solvent-dry coatings. Extra care should be taken during application of these coatings to prevent pinholes from forming.

Origin and Potential Causes:

• Improper surface cleaning or preparation. Moisture left on primer- surfacer will pass through the wet topcoat to cause pin holing.
• Contamination of air lines. Moisture or oil in airlines will enter paint while being applied and cause pinholes when released during the drying stage.
• Wrong gun adjustment or technique. If application is too wet, or if the gun is held too close to the surface, pinholes will occur when the air or excessive solvent is released during dry.
• Wrong thinner or reducer. The use of a solvent that is too fast for shop temperature tends to make the refinisher spray too close to the surface in order to get adequate flow. When the solvent is too slow, it is trapped by subsequent topcoats.
• Improper drying. Fanning a newly applied finish can drive air into the surface or cause a skin to form, both of which result in pin holing when solvents retained in lower layers come to the surface.
• Insufficient sanding or filling of pores in fiberglass substrates.
• Insufficient mixing of polyesters.
• Solvent popping that has not been sanded to smooth.
• Insufficient isolation of polyesters.

Prevention Techniques:

• Thoroughly clean all areas to be painted. Be sure surface is completely dry before applying undercoats or topcoats.
• Drain and clean air pressure regulator daily to remove trapped moisture and dirt. Air compressor tank should also be drained daily.
• Use proper gun adjustments, techniques, and air pressure.
• Select the thinner or reducer that is suitable for existing shop conditions. Mix polyesters.
• Completely sand smooth solvent pop pores and other defects before refinishing.

Remedy :

After thorough drying of the affected area, sand completely smooth, reapply polyesters or undercoats as necessary and refinish.

REFERANCES:

1.Ponderosa Protective Coatings, Causes and Prevention of Paint Failure, Date of access: 27 June 2016 http://ponderosapaintco.com/techdata/9.pdf

2.Northeast Automotive Paint, Pinholing, Date of access: 27 June 2016 https://www.autopaint18.com/html/pinholing.html