Why are commercial airplanes painted at all? While there was debate over why airplanes  are painted white and a question about how much a new paint job costed, what hasn't been asked is why airplanes are painted at all.

I remember one airline company sent out a press release in the 1980s or 1990s that keeping its planes unpainted (i.e, a nice silver) saved several hundred gallons of paint as well as considerable cost savings because the plane was also lighter and used less fuel.

If there are considerable positives to keeping planes unpainted, why do airline companies bother painting them? 

This is prompted by a recent flight I had on an US Air airplane that had just been repainted in the American Airlines livery. The captain joked that the paint was still so new that we shouldn't try touching it. Both companies are famous for always being in severe debt -- so saving every penny should have meant going for the 'naked' look.

Related: A quora question on how much money would be saved if airlines stopped painting their liveries and a Boeing white paper on painted vs unpainted
I remembered hearing that paint actually reduces the drag of the plane. Searching a bit I found this Boeing document  describing it.

So the painting reduces the fuel costs, instead of increasing them. Another interesting thing is, that planes are regularly repainted to keep the drag low. Of course there is the billboard effect too.

From the report:


  In response to numerous questions raised by 
  Boeing customers regarding the efficacy of 
  surface coatings to reduce drag, Boeing has
  investigated some possible airflow physics 
  explanations. Possible explanations postulated by Boeing aerodynamicists include:
  
  
  Increased regions of laminar flow due to reduced surface roughness.
  Reduction in surface roughness resulting in lower skin friction drag, when flow is turbulent.
  Reduction in dirt and/or insect adhesion resulting in reduced roughness and hence reduced skin friction drag.