Two of the more popular terms that one might come across while choosing the correct type of plywood are structural and non-structural plywood. The main difference between the two is the type of glue used in the manufacturing of the boards. There are two glues, interior (which does not stand up to wetting) and exterior (which is not affected by long term wetting and can be called waterproof).
Alternatively, plywood boards that meet structural standards fall under A and B-Bond types. They can definitely withstand some serious beating from the different elements like humidity! But, if you are looking for something that can withstand a lot of moisture, there is a special type of structural plywood called Marine Grade Plywood. All our exterior Street Libraries are built from Marine Grade Plywood.
Additionally, Marine grade uses exterior adhesive, which is the same adhesive used in the C-D graded plywood or any other exterior grade or “interior grade with exterior glue” plywood.
Non-structural plywood, on the other hand, falls under the C-D Bond. They are sometimes called interior plywood and is often used for aesthetic finishes inside the structure. The C-Bond plywood board has a higher-grade face and is your best option if you want it painted on. Applications of C-D Bond type include ceilings, wall linings, furniture, and decorative applications. They are not suitable for structural as well as outdoor use. The glue bond will deteriorate when it is exposed to wet and damp conditions as well as temperature changes.
However, good quality painting (at least 3 coats) will slow down water ingress and protect your CD rated (indoor) Street Library from the elements. So, yes you can use them outside, but be prepared to paint them very thoroughly with a high quality paint.