Whether relatively new or decades old, those railings must be clean, dry and smooth before applying a porch or deck railing paint. For any wood railings that aren’t freshly installed, wipe them down with a tack cloth if they’re relatively clean or a slightly damp rag. If the railings are really dirty, remove as much dust as possible, otherwise the paint won’t adhere well.
Once the railings are dry, sand rough spots with fine-grit sandpaper. Go over the railings with your hand while wearing a garden glove to find potential splinters or snags, then sand those away. Wipe the railings with a tack cloth again.
Apply painter’s tape around the bottom edges of the posts, then cover the area beneath and around the railings with a drop cloth or sheets of newspaper, taping down the edges of the paper to keep the sheets from blowing away. Use tape to cover all nearby areas that you wish to protect from paint, such as the surrounding wall area where walls and railings meet.