The Improper Behavior achievement in Dead Rising 2 demands that you spraypaint all of the 33 Zombrex posters hidden in the game. This take a lot of paint and lot of observation, and some help from this video. It will show you where each poster is, and the creator wrote up a handy guide to getting enough paint.
From the Creator:
There are 33 Zombrex posters to spraypaint. First things first you need to get spray. You can find green spray right in front of the maintenance room in the food court or in the entrance hall of the fortune city arena. Anyway, the easiest spray to find is the red colored in the store called "The Closet" left hand site when you leave the safe house. When you leave an area (so that the loading screen will appear) and re-enter it, all objects will respawn. You can make use of this little trick to get enough spray for all 33 posters at once. You will need about 6 sprays in total. The color doesn't matter, you can use any spray color you want.
All Zombrex posters can be spraypainted at any time and can't be missed. There is a bonus poster which can only be found during case 3-2 (in the Americana casino in the room where the mercenaries are in).