We bought our coop a month or so ago and I've been waiting for a break in the weather to paint the old, weathered plywood (fortunately it was exterior grade and NOT OSB so it has help up well, just old looking. I used a can of the "barn paint" from TSC and the wood was so dry and weathered that it just soaked it up - barely covered it using the entire gallon. In a few weeks I'll probably put another coat on.
Before:

After: (Will be putting tin signs back on tonight.)

Before:
After: (Will be putting tin signs back on tonight.)