You asked about the following sentence:
"I need a warning sign to post the anti-blast door in the 3rd floor of the packaging area."
I'm not too sure of your intended meaning, so you\ll have to choose the correct one depending on your meaning:
"I need a warning sign to post ON/ABOUT th eanti-blast door ON the 3rd floor IN the packing area."
Note:"post ON" would mean your're going to put the sign on the door;"post ABOUT" would mean you're going to post the sign somewhere in the area to warn people about the door.
Also, it's not too clear what's on the 3rd floor-the anti-blast door or the sign or both.
Perhaps the following would clarify your meaning:
"I need to post a warning sign ON the 3rd floor IN th epackaging area ABOUT the anti-blast door."