caulk sky is for when you want to texture the sky with your fog... caulk sky youd put on like an other skybox texture and you have to have a farplane value to use caulk sky...
the trigger brush has to have a trigger texture(you know that right?) and the if you cant go through it you either have a shader problem or you un-checked "non-solid on the surface properties window (press 's' if youv didnt know)that should fix it..