FINISH QUALITY

Just about anything and everything in and around the paint shop can affect the quality of the finished product in any given paint application process.  This discussion must therefore be limited to some of the more common paint shop maladies and those that are responsible for producing the greatest proportion of  imperfections in most paint application systems.  These are listed up below, along with descriptions, causes, and potential remedies.  It is our hope that this may aid the efforts of both operators and maintenance personnel in addressing their finish quality issues.

NA Paint can support you at any level in this.  We do not believe there is a resource in the world that can more efficiently or more quickly identify and correct problems affecting finish quality than North American Paint Applications.

Uneven Film Build Distribution

Variations in paint thickness across the part are not uncommon in electrostatic or conventional spray applications.  In some cases, these varying builds do not noticeably affect the quality of the finished part.  Sometimes though, heavy and light film builds do regularly and significantly contribute to runs, drips, sagging, mottling, striping, color mismatch, crazing, blistering, solvent pop, craters, fish eyes, peeling, and surface roughness.  If you cannot well control the paint distribution across your finished part, then it is likely you cannot well control the quality of finish on your product.

It is also helpful to understand that there is a real dollar cost, productiviy, and environmental impact associated with uneven paint distribution.  To illustrate, let us take an example of a 20 foot long extruded aluminum trim piece that is attached to an overhead conveyor and is painted in an omega style booth by an electrostatic rotary disk mounted on a vertical reciprocator.  Invariably, systems installed 20 years ago did not compensate for the changing fluid supply head pressures that occur as the reciprocator strokes up and down (changes in elevation of 22 feet with paint density at 11 lb/gallon is nearly 13 PSI).  The result is that paint flow rates vary with the height of the reciprocating disc.  They are lowest at the top of the stroke and highest at the bottom.  In these systems, it is common to see 20 percent variations, or more, in film build across the length of the part, even with flow control installed (but not ours!!!).

Because the part must be finished, film builds at the top are controlled at the point where 100% hiding is reliably achieved.  The rest of the part, however, has more paint than is required for 100% hiding.  In our example, where there are 20% variations, 10% of the paint sprayed per part is unnecessary.  If that 10% contributes to defective parts because of heavy build on the bottom of the part, then there is additional cost of scrap and rework.

Uneven film builds have many causes; two of the most common are 1) variations in the amount of paint delivered from the atomizer, and 2) uneven distribution of spray patterns across the part.  Both of these are easily correctable.  In the first instance, accurate and repeatable control of paint flow rates can significantly correct any variations.  In the second, spray patterns and atomizer positioning must be adjusted to synchronize with the part during the spray application.  If paint process parameters such as flow control, air pressure, conveyor speed, part location, etc., are controllable or repeatable, then this is a straightforward procedure.  

Dirt

Regardless of the method of application, dirt is the most generic and the most common defect found in paint finishes.  It can enter the finishing process at any stage prior to, during, or after painting.  For example, the target substrate can be contaminated prior to paint.  It is also possible that the paint can be contaminated.  The application process can even create the contamination.  And, anytime in the wet, dirt can impinge upon the substrate and blemish it.  

Dirt comes in many forms and it is the painters good fortune that it is usually visible.  A careful, methodical inspection of the painting process should reveal at which stage dirt is introduced into the painting process.  Once the source and type of dirt are identified, corrective action can be taken.  

Runs, Drips, and Sags

This category of blemishes can also be present in almost any system, regardless of the method of application.  These defects are also visible, either in the wet, or during or after curing.  Runs drips and sags are almost always caused by the application on the target of too much paint, either generally or locally.  Fortunately, there are a number of effective and efficient ways to control both the rate and the total of paint build-up on the part.

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 bicycle fender to the frame or a bumper to the hood.

On parts painted with solid color paints from the same tote or batch, 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 flat.

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.  They are 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.  

Peeling

Paint peeling refers to paint that does not long adhere to the surface.  It can appear in a wide range of geometries.  It is usually caused by incompatible foreign materials either in the paint or on the substrate, but can also be caused by incompatible substrate material, and occasionally improper paint application process control.

Orange Peel

Orange peel refers to a texture in the painted surface similar to that of an orange skin.  It is commonly caused by overly viscous paint, paint with very quick flashing solvents, or improper paint application process control.

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