Prepare the barn. Scrape away loose paint or debris using a paint scraper. Pressure wash the barn's outside to remove dirt, dust or other debris such as cobwebs, bird droppings and loose paint.
Allow the barn to dry completely. Select a day to begin painting when the temperature is above 50 degrees and there is no rain in the forecast for at least four days (or longer, depending on how long you think it will take to paint the barn).
Prepare the paint sprayer, following the manufacturer's instructions.
Apply the first coat of wall paint. Begin at one of the barn's corners and slowly work your way around the barn's exterior walls. Because this is the first coat of paint, make sure you spray a generous and uniform coating. Work your way around the entire barn, using a ladder to paint areas that are out of arm's reach.
Apply a second coat of wall paint. Because this is the coat that will be visible, take time applying the second coat, paying attention to detail.
Paint the trim. Hand paint the trim with a paintbrush. Use slow, careful strokes to give the paint a smooth, even finish. Avoid loading the brush with too much paint, which can result in unsightly drips.