That's the down side of making a coop out of sub par materials. Recycling is great, second use material is great, being resourceful is awesome... but sometimes when it comes to the value of your time and how long you want your structure to last, it is best to buy some stuff you know is good and will work in your climate. Here in the PNW I used t1-11 plywood siding for my coop because any missed spot that allows water to touch any press board material = certain water damage.
Good luck on your new walls! I used killz primer and two washable semi-gloss top coats of paint for interior and exterior.