Knowledge Bank




Paint Defects and Good Application Practice


Good painting practice is a set of procedures given by the manufacturers of Paints, pretreatment chemicals, application equipment and so on, for a painter to follow, in order to get the best results from Painting. The good practice further evolved by observing defects after painting under various ambient conditions, by people having various levels of awareness and knowledge. Investigating and eliminating the defects by improving certain parts of the procedures through experience and sharing them within the industry creates what is known as "Industry best practice on painting". All paint defects have one thing in common: a cause. The cause is always in missing some steps or not following the recommended procedure - somewhere along the line - from cleaning and pretreatment to paint application and curing.

Tracking down the cause of a paint defect ought to be straight forward. The secret of good "detective work" with paint defects is to build solid foundations of knowledge in good painting practices and in painting defects. A painting defect is like a sign along a highway - it gives information that leads to a goal - getting to where one wants to go. The defect always points to a deviation from 'good painting Practice' assuming that the paint, thinner, pretreatment chemicals and the substrate are according to expected standards.

A comprehensive list of painting defects could include perhaps several hundred names. Such a long list can be shortened considerably by limiting it to the major types of defects. By and large, these major defects come close to covering all types of defects. A brief explanation on the causes of the defects is followed by the 'good practice' which can help avoid the defect. The defects that will be covered are:

  • Blisters.
  • Bubbles and craters.
  • Color mismatch.
  • Dirt.
  • Fish eyes.
  • Gloss variations.
  • Mottle.
  • Orange peel.
  • Runs, sags and curtains.
  • Paint adhesion loss.
  • Soft paint films.
  • Solvent popping, boiling and pinholes.

Blisters

Definition:
A blister in a paint film is a small dome-like raised area that contains, or at one time contained, moisture (water, water vapor or both).

Causes:
Blisters on painted metal are traceable to contamination left on a surface prior to painting, either from water, residual soils, poor rinsing or incomplete dry off of rinse water. Fingerprints may contribute salts or skin oils that can lead to blisters. Any unrinsed cleaners, pretreatment chemicals or unremoved greases and oils can permit moisture to get under a paint film in a matter of days and form a blister. The moisture can also come from poor water removal in a compressed air supply, although when that occurs, it tends to cause a large, obvious flaw in a paint finish. Moisture in or under a paint film produces rapid blistering, expanding and contracting with temperature change and lifts the film. The moisture strains a paint film to the point that cracks may appear. Once the film cracks, moisture easily reaches the underlying surface, and corrosion can begin. Inadequate rinsing or rinsing with water containing high amounts of dissolved solids can sometimes cause a problem. When these difficulties are experienced, it may be necessary to use ultrapure rinse water containing as little as 10 parts per million total dissolved solids. The paint system being used is the major determinant of the dissolved solids that can be tolerated in rinse water. In some cases 200 to 300 parts per million dissolved solids may be adequate to avoid rinse water deposits. Tap water rinses may be followed by a brief misting with deionized water to eliminate blisters from this source. Blisters can be produced on painted parts by osmosis. Such a scenario is formed with a paint film when water soluble solid residues remain on the surface of a substrate beneath the paint film. When the paint film gets wet, moisture will seep through the paint film, contacting the residues, forming pockets of concentrated solutions. Osmotic pressure will draw concentrated solution sites. When a sufficient amount of moisture has been drawn from the top of the paint film through the paint film to the pockets of solutions below, the paint film will be lifted from the substrate at the solution sites in the form of blisters.

Good practice:
To prevent blister formation on painted metal, be absolutely certain that the surface is free of contaminants before applying the paint. Be sure that the gun air supply is clean and dry.

Bubbles and Craters

Definition:
A bubble in a paint film is a small dome-like raised area that contains (or earlier contained) solvent vapor. A bubble closely resembles a blister (which contains water and/or water vapor). A crater is a small, concave depressed area that formerly was covered by a bubble. The breaking of the bubble contributes to the crater's rounded bottom and built-up sides.

Causes:
Bubbles tend to form during a cure cycle when the top layer of the paint film skins over before most of the solvent in the film has had a chance to escape. The rising solvent pushes up portions of the skinned-over top layer, forming bubbles. If the bubbles break, the raised skinned-over areas constrict to the sides, forming craters. Bubbles (and craters) are usually traceable to the following causes:
An extra heavy wet film application. This can trap large amounts of solvent in the film. The bake oven heat may skin over the top layer of film before most of the solvent has escaped, forming bubbles.
A solvent blend that evaporates too slowly. This will invite skinning of the top layer of film before most of the solvent escapes, forming bubbles.
Insufficient primer cure. This can leave an excessive amount of solvent in the primer, which may form bubbles in the subsequently applied topcoat during the topcoat bake cycle.

Good practice:
The best ways to prevent bubble (and crater) formation are to avoid applying a coating in too thick layer, use the proper solvents and maintain specified intervals for applying topcoat over a primer.

Color Mismatch

Definition:
Color deviations from one part to another are shifts in the color of a paint film that occur in applying the same batch of paint.

Causes:
The causes of color deviation can include:

Inadequate agitation. Improper agitation of the paint can leave uneven pigment distribution, which will cause color deviation. The paint may not have been thoroughly agitated in its container before being put into the paint application circulation system. The pigment portion of nearly all paints will settle during use if the paint is not agitated at least occasionally throughout use. Inadequate mixing will result in the alternate application of resin-rich paint and pigment-rich paint.
Low film builds. Excessively thin films may fail to hide the substrate thoroughly.Some colors are far more effective in hiding than others. Light colorcoats often require a greater film thickness to hide a given substrate completely than do darker shades.
Different application procedures. Various application procedures can produce different apparent colors with the same paint. Paint applied with air spray may have a different shade than the same paint applied by a centrifugal rotary applicator due to different degrees of atomization and particle delivery velocities. Electrostatic versus nonelectrostatic application can produce color deviations as well.
Different substrates. Even if the same paint is applied with the same application device at the same time, the color may look different if the substrate material varies. If an assembly of plastic and metal is painted, a variation in the shades of color may occur. This is thought to be due to the different heat-up rates (heat capacities) of metals and plastics during the curing process. The paint on the plastic could remain wet for a longer period than the paint on the metal, or vice versa, allowing variations in the rates of pigment separation. This is less likely to be a problem with an air-dry paint than with a baked finish.
Different surface textures. Surface texture differences can cause perceived color deviations. However, color detection instruments may not indicate a color difference. No correlation seems to exist in the perceived shift of light and dark colors on smooth and rough surfaces.
Overbake (for force drying and stoving paints). Baking either too long at normal temperatures or at overly high bake temperatures can result in color variation. Some paints seem to be much more sensitive to this than others.

Good practice:
The prevention of color deviations requires absolute consistency in paint agitation, degree of film wetness, film thickness and application procedures. Color shifts with widely different substrates such as metal and plastic can be prevented in some instances by baking each substrate separately with different bake cycles. Perceived color shifts from part to part can be prevented by calibrating the baking conditions for each substrate painted with the same paint, other conditions remaining unchanged.