Possible ways to print soft skin for human robot?
Oh that is a tricky tricky question!

First you will probably need a good scan of the robot. You will have the cad of the robot but chances are scanning will make life easier.

http://www.3ders.org/articles/20151201-kinect-easy-3d-printing-tool-with-release-of-3d-scan-app-for-windows.html

Also see Reprap for other options

http://reprap.org/wiki/3D_scanning

Following... it is not going to be that simple. If it was my project I would
Print a mold out of ABS. ABS reacts to acetone and you can vapor treat it.

http://www.logarithmic.net/pfh/blog/01366106156



That said you might burn your house down. Also it is inconsistent. But would give you a smooth finish. 

Next you can do just normal mold printing and be okay with the ribbing. Then you pour your silicon material



Next you can well just print with silicon. The ninjaflex guys have a ton of different types of flexible materials and new ones coming out. 

https://ninjatek.com/products/filaments/ninjaflex/



On that topic I will mention a local company that I have no direct affiliation with. I just see them at the hacker space. They make a extruder just for flexibles. 

https://flexionextruder.com/

Other areas to look at. More for making metal but it may be of use.

http://www.instructables.com/id/From-3d-printed-part-to-metal-the-lost-plaabs-me/

Last is again the mold but you make the molds on a expensive form 1. Or you make the skin on the form1 as they can do flexibles. Small build area. Material is around 100 a ltr or more

http://formlabs.com/products/3d-printers/form-1-plus/

Good luck!

Also check out this facebook group (no affiliation) 

https://www.facebook.com/groups/3DPrintProps/

If anyone has done this it is those guys. Oh Wait No I take that back Look into these fellows. They have a very active group. (google group) not to mention that they are also directly doing what you are.

http://inmoov.fr/




Just don't be like this guy and make it look like a movie star... 
http://www.mirror.co.uk/news/weird-news/man-builds-scarlet-johansson-robot-7667715