‘Throw-up’ or ‘Slap-up’ Stickers are commonly made from postal form stickers and delivery stickers free at the USPS Stores. Stickers can be coated or uncoated and come with a variety of adhesives. You generally need to remove most of it by scraping with a metal or plastic scraper and then brushing on Sensitive Surface Graffiti Remover generously, and agitating until the adhesive begins to dissolve. 

For uncoated paper stickers such as the ones found in the postal service, it helps to flood coat with  Sensitive Surface Graffiti Remover first and leave it on for several minutes before scraping it off. Repeat the process as needed.

If possible, steaming HOT water from your pressure washer is great for removing the paper, and even coated stickers will flap off once the hot water and steam get underneath. 

If you are dealing with uncoated sensitive STOP signs, then the sticker residue can often be cleaned off with Windex (window cleaner or something similar) before wiping away with a damp cloth. 

If you are able to access a heat gun, these can be very effective on coated stickers making them much easier to peel away. 

Sensitive Surface Remover or  Safewipes will remove any remaining residue easily. 

Tips for the Pros! After removing stickers or graffiti from faded powder coated municipal boxes, you can clean the rest of the box easily with a decent detergent (painters soap/ sugar soap trisodium phosphate / TSP) and a broom. Apply, agitate and rinse. If you just deoxidize a small area it looks pretty amateurish so allow for this in your quote and leave the box looking great!