Originally posted by Kunzite As macTak said, the external shell is magnesium alloy (in large part)
The internal chassis is stainless steel - don't even think to argue about this, it's well documented on their websites. And I can certify the body feels different than the (plastic shell) K20D, much colder in winter.
Of course, it's painted. The silver version will be painted as well, I'd guess.
Ok, I admit I cannot definitively prove that it is plastic, without tearing it apart, but I have very strong evidence from an unlikely source: the lens hood of the 18-135.
On the outside of the hood there is a substance that is
identical in texture in every way to the external shell of the K-5. This substance is distinct from the plastic material that makes up the structure of the hood itself. It is visibly separate when viewed from the edge on. This layer is
extremely thin. About the thickness of a sheet of paper by my estimation. This layer serves no purpose other than to give a certain texture to the outside of the hood, while the main structure is made up of a very different plastic with completely different texture. It is also not paint (in the traditional use of the term), because it has a definite, solid, hard edge to it.
Yes, as macTack accurately describes, this material is integral to the structure it is applied to (cannot be separated without damaging it), and is so thin it might as well be paint, but it is clearly a separate plastic material.
I am almost certain this is the same material used to cover the entire body of the K-5 to give it consistent texture between the back panel (undeniably solid plastic) and the metallic body. It is extremely difficult to make paint have perfectly the same texture as plastic. It is much easier to make a consistent texture by coating every surface with a very thin layer of the same substance (essentially "painting" with plastic), while letting the base material (metal or stronger plastic) handle the structural integrity. A plastic layer is also more resistant to scratching or damage than paint, and provides a slight "buffer" to direct damage to the substance underneath.
Whatever the technique, it is clear that the outer surface of the K-5 is not bare metal. A layer this thin (~0.1mm thick) would do little to the thermal conductivity properties of the base material, and would easily feel as "cold" as the metal underneath as you have clearly noticed.