This is one of the biggest mistakes people make before painting, regardless of whether they’re working on the interior or exterior. If you want the paint to adhere well, you need to apply it to a surface that’s clean, dry, and neither flaking nor peeling. Depending on the existing paint’s current condition, you might need to perform a considerable amount of prep work beforehand.
To start, you’ll want to wash the surface with a hose and a scrub brush with water and detergent—or you could use a pressure washer for quicker results. For removing loose, flaking paint, you’ll need a scraper. A disc power sander or random-orbit sander will help you smooth down the surface. Start by using 60-grit sandpaper, then end with 100-grit sandpaper. You don’t want to remove all the paint, just the parts that are loose or damaged. If there are any cracks or holes in the surface, make sure to fill them using a putty knife and wood filler. After the filler dries, sand down the area again. You should also make sure to brush off the dust, caulk the joints, and let the caulk dry before you start applying the primer and paint.
You’ll also want to prepare the rest of the exterior by covering decks, shrubs, gardens, patios, walkways, and everything else in the immediate vicinity with drop cloths or plastic sheeting. This step will save you from having to do massive amounts of cleanup afterward or potentially ruining certain items permanently.