When making signs you pretty much have to do it using brushes.
A common and easy technique is to make it a thin brush and tie it to
func_detail and set Passable to Yes (zhlt_noclip 1). This prevents it from both chopping the walls it's placed on, as well as generating unnecessary clipnodes the player might collide with.
Another technique is to actually embed it in the wall and either use the Clipping Tool to cut out a hole for the sign:
or, a bit more advanced technique is a sort of mix of the two. Embed the sign in the wall as with the second technique, but instead of clipping a hole in the wall leave it as it is and tie the sign to a func_detail again. This time set the Detail level (zhlt_detaillevel) to 1 and the Priority when faces overlap (zhlt_coplanarpriority) to 1 as well.
It will do a lot of Z-fighting in the editor (flickering) but in-game it's the sign that will be visible without any Z-fighting (because of the zhlt_coplanarpriority keyvalue).