Re: Prepping wood for a barn sign?
A good quality house paint over Kilz or Zinser primer will last for decades inside the barn and close to that outside.
House paint was created to successfully face the worst Ma Nature can throw at it - snow, rain, humidity, sunlight, etc. It will outlast any artist's paint under any type of finish/sealer - and doesn't need any. Computerized color matching is simple, too. Usually the smallest container size is the quart.
Practice freedom religiously, religion freely, and always stand up for the people who can't.