Yep, I just gotta ask
there's a way to put nodraw on all surfaces that don't need to be seen, and I don't know it. Someone care to help me out or link me to a thread with the answer?
You just go into the Texture Editor, and select the side you want to nodraw, select the texture, and click apply.
Wait, did I just answer a question?
OHSHIT PARADOX
First of all, keep in mind that any surface of a world brush that is either A.) facing the void, or B.) is completely covered by another world brush, are automatically not rendered. It's not necessary to put nodraw on every face that is facing the void since they are removed anyway.
To answer your question, no. How could there be? How is Hammer supposed to know what is or is not "supposed to be seen"? I might nodraw something that you might not, or vice-versa. There is no easy tool for this - break out that Texture tool and get to work. 
Wait, did I just answer a question?
OHSHIT PARADOX
His question wasn't how you apply nodraw to a surface, his question was how to automatically apply it to all faces that are not seen by the player.
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Oh.
I shouldn't be saying this, but here goes:
There is no substitute for hard work.
Pretty pointless though...if you want to be lazy just leave them whatever texture they are...not like it matters.
Boy is that not true! Maybe in Portal where the maps tend to be smaller and there is a lot less rendered at any given point, but nodraw'ing faces that do not need to be rendered is NOT pointless. You can save some serious budget if you are smart about it. Load up some HL2 maps or TF2 maps and see how smart nodrawing can save your life.
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So, nodrawing non-seen faces makes loading faster?
agree *1
but aside from just selecting multiple surfaces there is no way (as sleeper said) and like he said there's really no need, the only reason i started doing it is cuz when i first got into portal mapping all the tutorials and sample map said to do it. so i thought that (for portal) it might be important. once i relized it was like any other bsp based mapping engine, in that "If there is no way to see it i ain't gunna render it", then i stoped doing i. the ONLY reason i would see in doing it is so that you, the mapper, could differenciate from different areas. Like if you are outside and have some parts of the map turned off (using the vis groups) you can tell if you are still "inside" or "outside" [rambling complete]
[edit]
Wait what? i need to nodraw them? the outsides? what? or just stuff INSIDE the map that isn't rendered?