I love a pressure washer for cleaning.
While Killz paint is great, for coops and barns I use whitewash. It's cheap, kills bugs, germs and odors and looks good. Some of it will dust off and a new layer yearly is a good idea, but that new layer will take down some dirt and disinfect - and yearly disinfecting is never bad.
One part (coffee can) fine livestock salt, 3 parts barn/hydrated/masonry lime (NOT garden lime) and enough water to give it the consistency of fresh milk.
It goes on almost clear, and is a lovely, clean, bright white by the next morning.