To the right of the walkway entrance between Slot Ranch Casino and the Food Court, near the bar.
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Location description
In the area where Chuck battles Seymour during WWJWD?, the poster is on the wall on the north side, near the entrance to Maintenance Room 27.
On the opposite side of the fountain to where Chuck rescued Terri and Willa in An Industrial Fashion, there is another set of scaffolding against the wall. The poster is at the top of the scaffolding.
On the wall under the scaffolding diagonally opposite from the Maintenance Room 24 near the Arena exit.
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Location description
Bottom of the escalators as the player exits the casino to the Silver Strip.
When spraying the posters, ensure that Chuck continues spraying the poster until the 1,000 PP bonus is granted. If Chuck stops early, or get interrupted, the poster will not count. If this happens, just continue to spray the poster.
It is a good idea to note what posters Chuck has sprayed or do them in a defined order because the paint will vanish from Zombrex posters once Chuck moves into a new area.
The Zombrex poster in the cashier pen in the Americana Casino is not required for the achievement/trophy unless you have completed the case file.
It is also worth noting that you will only be able to access the second Zombrex poster in the arena area, listed above last, after completing the first case, otherwise the door will remain locked.
It is recommended to attempt this achievement if you are wearing any part of the ninja outfit, as zombies are less likely to attack you.
Spraying all posters will give "All Posters Bonus" worth 300,000 PP.