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I need to replace two pieces of exterior trim on a shed. Should the trim be primed and painted on all four sides or just the exposed sides. Years ago I was told to just paint the exposed sides so the wood could "breathe". Thanks for any help.
You do not need to prime/paint all sides, but you do want to make sure you prime/paint any exposed surfaces, which includes the bottom end closest to the ground.
Traditionally, siding on a home would be back primed to protect against moisture generated in the house from working its way from the interior to the exterior. Moisture entering the wood siding can cause peeling when the heat of the sun beats on the siding come spring. Moisture condensing on the rear of the siding can also cause tannen leaching from cedar,redwood siding or even fir siding. This causes yellow/reddish streaks to appear on the front side of clapboards.
However, a shed does not generate moisture in the same fashion as a house. No one is showering, cooking or doing laundry in that shed, so the same need to backprime does not exist. If however, the boards are very dry (under 8% moisture), priming all sides would not hurt. It is always a good idea to seal end grain.
Thank you so much for your replies. It's my sister's shed and she would be more comfortable if I primed and painted all sides, which I will do.
There's nothing to add to the former execellent answers, except: make sure that no rain or other water gets into the unprimed portion of the trim. Caulk the top and sides of the trim to make a good seal.