Final Surface Finishing

When a customer’s damaged car is brought to your shop, the objective is not to just repair it; rather, the car’s body component(s) must be as new when the repair work is completed. To achieve this goal, final surface finishing must be done. Granted, in some cases, not much work will be required. A little buffing and a little polishing, but, a thorough inspection is necessary to find and fix any imperfections. This will ensure that the finish is perfect. After all, the reputation of your business is on the line.

If any imperfections are discovered during the final inspection, those areas must be fixed before the vehicle is returned to the customer. We’re going to examine several final surface problems and detail the cause, some tips to repair it and some pointers to prevent it from occurring again.

POOR COVERAGE

Poor coverage is defined as the original finish or undercoat being visible through the topcoat.
Cause – Among the causes for this problem are the color not being thoroughly stirred or agitated, the color being over-thinned or reduced, the use of the wrong undercoat, or an insufficient number of color coats was applied.
Repair – There are two simple ways to correct this problem. First, apply additional coats of color until adequate hiding is achieved. Second, sand the finish and apply a similar colored undercoat, then refinish.
Prevention – Stir or shake your paint material thoroughly to ensure that all pigment is in solution/suspension. Don’t forget to thin/reduce according to the paint manufacturer’s directions. Use an undercoat that is similar in color to the topcoat and spray until hiding is achieved.

DUST CONTAMINATION

Very simply, dust contamination is dirt in the finish. As you can see in Figure 1, dirt particles stick to a wet finish. As a result, these particles become encapsulated when the clearcoat is applied.
Cause – There are several causes for dust contamination in the final finish. These include poor cleaning of the surface before painting, a dirty spray booth, poor air filtration in the booth, unfiltered air entering the booth and using dirty spray equipment. All of these reasons are the result of lack of proper maintenance.
Repair – The only way to repair a finish damaged in this manner is to sand it. If the amount of dirt is minimal, you can sand with 1200 or finer grit, then compound and polish to restore the gloss. If there’s a lot of dirt, you’ll have to sand the finish smooth and refinish it.
Prevention – The first thing to do to prevent dust contamination is to thoroughly clean the vehicle that will be painted. Then, clean the spray booth. Also, be sure you are using the correct air filters in your paint booth. Never use residential furnace filters. Be sure all spray equipment is clean. Finally, use proper masking material. Don’t use newspapers to mask as wicks from newspaper can break away and land in the wet paint.

RUN/SAGS

A run occurs when the coating (paint or clearcoat) does not properly adhere uniformly across the undercoat. As a result, the coating slips down off the undercoat or old finish, leaving the characteristic run marks (Figure 2).
Cause – One of the main causes of runs and sags is the result of poor spraying techniques. These can include holding the gun too close to the surface, using a gun speed that is too slow and double coating. Also, over reducing and/or using a thinner that evaporates too slowly will cause runs. Improper spray gun set-up or an unbalanced spray pattern can also lead to this problem.
Repair – The wet paint film that has run has to be removed. If the finish is dry, remove the excess paint by block sanding with a 1200 or finer grit sandpaper then compound and polish to restore the gloss.
Prevention – Runs and sags can be prevented by using proper spray techniques, allowing sufficient flash times between coats and mixing the coating according to manufacturer’s directions. Also, use the recommended spray gun and proper fluid tip and air cap combination.

ORANGE PEEL

Orange peel is a another common problem. The paint finish displays an uneven texture that looks like an orange peel, thus the name.
Cause –Orange peel often results from improper spray techniques and spray gun set-up. Also, orange peel occurs when heavy wet coats are applied in a short period of time. Finally, thinner/reducer that evaporates too fast will cause orange peel.
Repair – Minor orange peel can be removed by polishing, but in many cases, you’ll have to sand the finish smooth, then compound and polish to restore the gloss. In extreme cases, sanding the finish smooth and re-finishing is required.
Prevention – Use the proper reduction ratio and spray at the correct air pressure to prevent orange peel. Select the thinner/reducer based on 
temperature, humidity, air movement and the size of the repair. Also, avoid applying heavy coats that produce thick films. And don’t forget to adjust the gun for the best atomization.

MOTTLING

Mottling is the striped appearance on a finish with a metallic color. It is also known as streaking and tiger/zebra stripes.
Cause – Basically, mottling is the uneven distribution of metallic flake. There are several reasons, including the following: using a spray gun that produces an uneven pattern (Figure 3); the use of an improper spray technique (tilting the gun or holding the gun too close to the surface); and, an uneven spray pattern overlap. Also, mottling will occur when the clearcoat is applied to a basecoat that has not properly dried.
Repair – If the finish is still wet, apply a higher pressure mist coat, panel by panel. Or, you can allow the basecoat to flash, then apply a low pressure mist coat. Unfortunately, if the finish has dried, it must be sanded and refinished. It should be noted that large areas of basecoat must have clearcoat applied before sanding. Small areas of nibs, though, can be removed by wet sanding.
Prevention – Use the recommended spray gun, including fluid tip and air cap the material being sprayed. Always adjust the gun for best atomization and balanced spray pattern.

METAMERISM

Metamerism is the effect in which one color appears to be the same shade as a second color under one set of conditions, but appears to be different under 
a different set of conditions. This is most common when two colors match in daylight, but look different under artificial light, especially sodium street lighting.
Cause – Aside from the fact that people can perceive color differently from one another, there are some physical reasons for this effect. The most common is a difference in the pigment and/or base color composition between the original paint and the repair paint. This occurs when no formula exists for the original paint.
Repair – Slight variations in color due to metamerism can be eliminated by blending the repair color into the surrounding original color panels. In extreme cases, the repair color must be re-mixed or assessed by electronic color measurement.
Prevention – To prevent metamerism for unknown color shades, use electronic color measurement. For known color shades, check the color under different light sources. Tint only with base colors which form part of the mixing formula.