Lay tarps near the working area to catch dust and debris, then put on your respirator mask.
Scrape off old paint from the porch, if necessary, using a hand paint scraper. Collect chips as they fall off for disposal.
Dispose of paint chips and dust collected by hand or on the tarps as directed by your local sanitation department. Unknown paint can contain dangerous levels of lead, so do not dispose of this in household garbage.
Clean the porch with a garden hose. If areas are moldy or very dirty, add 1 cup of bleach and 1 cup of trisodium phosphate detergent to a gallon bucket of water and scrub these areas clean with a stiff brush. Allow the solution to sit for 30 minutes, then rinse well. Let the wood completely dry, which, depending on your climate, can take a few days to a week.
Fix any cracks in the porch wood using a wood-epoxy putty. Follow the manufacturer's instructions for application and setting time. Sand dried epoxy putty with 60-grit sandpaper.
Apply an oil-based primer with a paint brush.
Apply latex-acrylic caulk around the trim and allow to dry, following manufacturer's directions.
Pour some exterior paint into the paint tray. Roll paint onto the roller, and then apply to the porch in even, slightly overlapping strokes. Allow the paint to dry thoroughly between coats.