Accurate and repeatable control of paint application rates can significantly improve this problem.  All aspects of the applicator including size, head to target distance, bell speed, spray pattern size and distribution, voltage levels, dwell time, and angle of application can and ought to be used to control finish quality.  The composition, shape, motion, and orientation of the target ought to lend itself to the process in the most accommodating way.  Booth conditions can be a contributing factor.  Finally, the applied material must be suitable for the process.

MOTTLING, STRIPING, AND COLOR MISMATCH

Mottling, striping, and color mismatch are all terms used to describe color variations on the finished part.  The term mottling is generally used when referring to color variations across a single part.  Often these variations in shades of color are random.  Sometimes, however, there is a distinct pattern and repetition to the color variations.  These can be described as striping or checkerboarding.  The term color mismatch is generally applied when comparing the color of two whole parts such as a car door to the body panels.

The level of pigmentation in paint affects color.

On parts painted with solid color paints from the same tote or batch and have consistent solids suspension, any color variation is almost always caused by film builds that are below the level at which 100% hiding is achieved.

On parts painted with metallic paints from the same tote or batch, film builds that are below the level at which 100% hiding is achieved is but one possible cause of color variations across a single part, or among different parts.  Possible causes of color variation with metallic paints include variations in film build, inconsistent fluid delivery, spray pattern non-uniformity, paint impact speed, improper atomization, electrostatic effects, and metallic flake damage.

To help understand the cause of color variation in metallic paints, it is convenient to visualize the metallic flakes as tiny strips of aluminum foil.  When these strips are laid flat on a surface they reflect light.  When these strips are perpendicular to the surface, standing on end, they reflect very little light.  If part of a surface has the strips standing on end, and another part has them lying flat, there will be a noticeable difference in color.  In other words, variations in the orientation of the metal flakes relative to the surface is a cause of color mismatch in metallic paints.

Generally, the most desired appearance of metallic finishes is when all the flakes are lying somewhat flat, some tri-coats the possible exception.

So, variations in film build affect the metallic flakes by providing 'deeper pools' of wet material in which the flakes can reorientate, by electrostatic or other forces.  Metallic applications generally require a sufficient paint impact velocity to flatten out the flakes on the surface.  This is why air-atomized guns, which apply paint at higher speeds, have traditionally been used.  Metallic flake damage is often caused by the paint supply and recirculation system and is discussed more fully in the chapter on paint supply sytem sheer.

CRAZING

Crazing describes small cracks that appear in the finish.  Generally, these cracks are quite numerous and have random axis, resembling a dry lake bed.  They are akin to the crazing seen in pottery glazes.

Crazing is caused when two materials bonded together expand or contract at different rates causing lateral surface forces greater than the strength of bond.  The expansion and contraction can be caused by thermal, chemical, or mechanical forces.  Crazing can appear at any adhesion interface including that between substrate and primer, between primer and basecoat, or between basecoat and clear coat.

It is often easy to determine which adhesion point cracks and whether or not the cause is thermal.  If it is thermal related, as usually is the case, it is often easy to determine the conditions under which the crazing occurs, e.g. which stage of heating or cooling.  Usually, thermally induced crazing is controllable, either by less abrupt temperature changes or by using materials with more closely matched coefficient of expansions.

BLISTERING, SOLVENT POP, CRATERS, and FISH EYES

These terms are often used to describe relatively small circular blemishes in the finished surface with a void of paint.  Although they are sometimes interchanged, the following definitions are widely accepted.

A blister is a raised area in the dry paint finish with a hollow center that is crusted over, or was crusted over with paint.  Foreign materials painted over on the substrate can cause blistering by evaporation during curing.  Solvent pop is closely scattered small blistering caused by solvent evaporation during curing.  This occurs when conditions do not permit adequate flashing of solvents, either locally or across the finished part.  Craters are spots where the paint on the surface, once coated, has receded, often concentrically, to leave a spot with no paint.  Cratering is caused by incompatible foreign materials either in the paint or on the substrate.  Fish eyes are craters that have a bit of material remaining in the center.