The colors and symbols used on an aircraft are instantly recognizable. At this moment, even mentioning the name of American Airlines, Mango Airlines, or Southwest airlines brings an assortment of imagery and colors to my mind. The design elements on an aircraft can take an airliner from common to iconic in an instant. As such, when making the decision whether to use paint, polish, or both, airliners must consider marketing, maintenance, and overall cost.
An aircraft is a visible identifier for an airline. As a result, most of the decisions made about paint are based on marketing factors. While some airliners prefer bright, engaging colors and patterns, others go for the sleeker industrial look of the bare aluminum airframe. Either way, airliners want their aircraft to maintain a look that creates the perception that they are new, and therefore safe.
And this is where aircraft polish really comes into play. Polish is used on painted and unpainted aircrafts, both. Some airlines choose to keep the aluminum airframe exposed, i.e.: Cathay Airways Silver Bullet freighters. These aircraft will only utilize paint for critical aircraft fixtures, (all airlines use light grey paint to designate these features) and branding. If the aluminum airframe is unpainted and exposed to oxygen, it goes through an oxidation process. While not harmful, this process makes the aluminum look dull and weathered or even rust. In order to remedy this, an aircraft of this nature needs to be re-polished and buffed up to 3 times a year and is also washed twice as often as a painted plane to remove oxidation build up.
This brings in the element of the overall cost. Operators will need to consider overall cost due to maintenance and corrosion protection when considering what paint and polishing method to use. Both methods have different associated maintenance requirements. Most airliners repaint their aircraft every 4 years, during a scheduled C-check or D-check. Costs involved with repainting include labor, primer, aircraft wax products, and more. Painted aircraft must also be checked for cracks or chips which can collect dirt and moisture. Worm corrosion is a particular worry— hydrogen is released between the metallic surface and the paint, creating extensive lifting of a paint layer.
If an aircraft is polished, this involves the added costs of the polish itself, mechanical buffers, more frequent washing, and labor. Areas of oxidation will also on occasion need to be buffed out to prevent further corrosion, which adds to their overall cost. Some argue that aircraft that are simply polished, and not painted at all, are more fuel efficient. This is due to the overall reduction in weight, as paint can add 300 - 600 lbs. of additional load. However, this has been debated across the aviation industry as paint has also shown to reduce drag by providing a smoother surface. Some aircraft will account for potential excess weight by not painting the underbelly of the fuselage.
Overall, the net operating cost of polished aircraft slightly exceeds that of a painted aircraft due to the frequency of maintenance. Most airliners utilize a combination of both paint and polish methods to maintain the look and efficiency of an aircraft.