How to clean painted surface?

My front porch got reshingled today and new wood gutters ('cause the house is 120 yrs old) will go up by next weekend. The neighbors seemed pleased when the crew informed them what was going on. Next thing, I want to repaint the white railings and columns that support the porch roof
I believe it is considered best to paint a clean surface. What might be considered a good cleansing agent for outside painted wood surface? Lowe's is only two short bus stops away, so I should have a good range of product available.
David Ames
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Congratulations on the improvements.
Use Tri-Sodium Phosphate (TSP). Follow the instructions on the label. Wear rubber gloves.
TSP is not too hard on plants, but it's still good to protect them: * Wet down the plants. * Cover them with plastic. * Clean the porch. * Remove the plastic and wet down the plants again.

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