Check out this CATIA V5 tip: http://www.ignitecast.com/p/bV3vXBR4Oo/
I created and uploaded this Ignitable to IgniteCAST.com to show you how to extract a surface from a solid in CATIAV5. I will also explain why the edges of the extracted surface are sometimes joined. This tip is for intermediate CATIA V5 users using the Generative Shape Design (GSD) and Part Design workbenches, so I hope you find it useful.
Steve