Having space on a building that could be drawing in customers and not using it is a waste.
Exterior building signs may be the most effective advertising you can buy. Every business needs to be known to their customers. How to do that depends on a number of factors. If your business depends on location and your location needs to be seen, then you need an exterior sign.
A sign above the door or a sign on the door invite passersby in. It makes your business more approachable, makes it easier for