Sandwich boards are made from high density polyethylene which means that discoloration is an unlikely occurrence
. This also means your sandwich board is made of some highly durable material and can take a beating. However, if you have a vinyl sticker sign on your sandwich board you are going to want to clean your board regularly to make sure dirt particles do not get under the edge of the vinyl stickers. Otherwise the surface could start to peel off which in no way looks appealing. A simple rubdown with a wet cloth around the edges of the sticker will go a long way toward making sure your board stays in the best condition.
Secondly, sandwich boards are weighted in order for them to hold up against the wind and other factors of the environment. Eventually, they'll still get knocked over by either people walking outside or huge gusts of wind. No worries, just try to minimize the appearance of scratches by cleaning out the scratched up areas so they're not discolored. The occasional scratch is unavoidable. Making sure that the actual sign portion is in good shape is probably most important as it is most likely to take a beating from mother nature.
After you are done with business for the day, be sure to bring your sandwich board back inside. Anytime that you can keep your board inside rather than outside will increase its life a little bit and also reduce the risk of theft. If you have vinyl stickers, they actually have a higher chance of peeling and getting damaged by the weather than your sandwich board. These are all pretty simple tips for sandwich board care, but added up over time they can pay off. Best of luck with your restaurant business or any other business you might be involved in!