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KTA-TATOR, INC.
Corporate Headquarters
115 Technology Drive
Pittsburgh, PA 15275
Phone: 412.788.1300
Fax: 412.788.1306
E-mail: info@kta.com

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KTA Challenge #10
Answer - Overspray


Photograph 1 - Challenge microphotograph at 200x	Photograph 2 - Overspray at 30x	Photograph 3 - Surface free of overspray at 30x

Challenge 10 (Photograph 1) is a digital microphotograph (200x) of an oversprayed coating. It can be more easily recognized in Photograph 2, which was taken at 30x. As a point of comparison, the appearance of the same coating at 30x without overspray, is shown in Photograph 3.

Overspray occurs when atomized droplets of coating do not flow and knit together to form a continuous film. It is typically caused by improper spray gun adjustment or when the gun is held too far from the surface. The net effect is that an insufficient number of droplets reach the surface to allow for full coverage and/or the droplets loose solvent as they pass through the air, inhibiting their ability to flow together to create a continuous film. It is possible, however, for the individual droplets to exhibit good adhesion to the underlying film, as is the case for the coating depicted in the Challenge.

Overspray often occurs when painting adjoining surfaces. For example, when painting a steel shell with a number of brackets attached to it, a good wet coat can be applied to the shell and the atomized droplets flow and knit together to create a continuous film. But when the brackets are painted, some of the coating misses the brackets, potentially creating overspray on the shell. If the overspray lands on an underlying wet coat rather than dry coating or bare steel, coating life may not be compromised if the individual droplets fuse into the coating and exhibit good adhesion. But if the overspray "mists" the bare steel, unless additional coating is applied to it, the primer will be non-continuous, and premature rusting may occur. If the overspray occurs during the application of the finish coat, aesthetics may become an issue. This is often seen when localized spots are touched up by spray. The center of the repair area is smooth and glossy, but the perimeter is dull and pebble-like, due to overspray at the edge of the spray pattern. While corrosion protection may not be jeopardized, aesthetics could cause the repair to be unacceptable.

At some point, the distance between the spray gun and the surface can be so great that overspray becomes dry spray. In the case of dry spray, the atomized droplets are dry when they reach the surface, creating a loose powdery film that does not knit into the underlying coating or substrate. Dry spray can interfere with the adhesion of the next coat and should be removed prior to subsequent applications.

KTA can assist you with application questions and provide all of the instruments needed for measuring coating thickness, determining coating adhesion, and assessing film continuity.

This one turned out to be more difficult than expected - there were no correct answers. The majority of the "coatings" answers involved sanding discs and sandpaper, or non-skid coatings. A specific brand of coating was even mentioned in one case. Others guesses included outgassing, rust bloom, a poorly applied coating, the backside of coating disbonded from a blast cleaned substrate, and phase separation of the coating. A few "non-coatings" answers were also provided - candy coated peanut, tomato sauce, and a slice of pepperoni. Since no one correctly guessed overspray, every answer (even pepperoni) was eligible for the prize. By luck of the draw, the winner is Kevin Kress of First Energy Corp., who guessed that the Challenge was a microphotograph of an abrasive sanding disc.