Effect of paint on drag force Aerospace engineering as well as automobile engineering gives a particular significance to the shape of a vehicle to enable proper and more effective transportation.What I want to know, is, about the impact that the protective covering aka the paint on an automobile etc. has on the drag force.While I am assuming it is not much, but, can our using a more slick coating, say Teflon paint,have an impact on the motion, thereby increasing fuel economy. Do let me know the quantitative figures involved.Thanks.
 A: Teflon has a low surface tension. What counts in viscous drag is the mechanical roughness of the surface. If the "peaks" of this roughness stick out into the boundary layer, they decelerate the flow more than a smooth surface would.
Normally, the drag coefficient drops with increasing Reynolds number. The Reynolds number is the ratio of inertial and viscous forces and can be calculated by dividing the product of flow speed and length by the kinematic viscosity of the fluid. Depending on the relative roughness (roughness relative to length), at some point the drag coefficient stays constant with a further increase in the Reynolds number. Only then will roughness on the scale of different paint surfaces become a factor in drag.

Friction drag of tube flow for different values of roughness (picture source)
German air force units in WW II would routinely test-fly new attack airplanes and select the ones with the highest top speed and then polish those. Normally, the camouflage paint on the aircraft was matte, but when polished, they would fly a little faster. Those airplanes were selected for missions where top speed would be important.
Roughness on the scale of rivet heads or larger is another matter. Here, the drag effect can be felt at all speeds. Therefore, most rivets are flush on aircraft.
A teflon coating would only be helpful if the structure is soiled by insects. Gliders have a serious problem on summer days with bugs which trip the boundary layer early and increase drag.
