Many times customers will call and ask us to make decals to put on signs that they already have up. This is often being requested because the customer needs to change an address, name, ect. There are several things to keep in mind when considering the purchase of decals to apply to a preexisting sign.
Typically decals will adhere to an aluminum sign better than a plastic sign especially if the sign has already been hung and has been out in the elements.
The area on the sign that the decal is being applied to needs to be extremely clean. We usually suggest cleaning the area with rubbing alcohol. If the area is not clean and free of dirt and grease you will not get good adhesion.
It is also important to keep in mind that the decal is not going to last as long as the sign. At some point you will probably need to replace the decal with a new one.
If you are considering decals on a posted sign because your imprint is no longer valid we typically suggest against it. If your posted signs are on the light gauge material, depending on the quantity, many times it is less costly to just have the entire sign reprinted. It will typically cost more for the decals than it will to reprint the sign and you will have a better product.