Removing old paint from houses and other structures can be a chore. While it seems to be just flaking away, trying to remove it is the quickest way to find out how well it may be holding on. One of the keys is to understand that total removal is probably not possible and is typically not the goal. In prepping for a new paint job, the main objective should be to remove any loose or built-up paint that will interfere with application of the new paint.
Scrape a small area with a stiff brush to see how the siding responds. For hard wood that doesn't scratch, go with wire bristles. For softer wood, use a stiff nylon bristle. Some brushes have the bristles directly below the handle and afford maximum pressure, others have long handles that provide maximum reach. Try one of each.
Brush over the entire surface of the structure with firm, heavy strokes. This will remove large patches of loose paint and help loosen more stubborn areas. Focus your work on sections where you can see the edges of the paint lifting away from the siding. This will help reveal areas that will need more work.
Apply chemical stripper after brushing if the house has multiple layers of paint and getting to the wood seems difficult. Apply a gel-type stripper recommended for exterior use according to label instructions. Typically it can be applied with a paintbrush. Use gloves and eye protection for comfort. Allow adequate time for the stripper to act before scraping. Rinse and allow time to dry before applying heat or sanding in areas treated with stripper.
Scrape in areas that have lots of edges on the remaining paint. This is an indication of multiple layers or that the paint is becoming brittle and will not make a good base for your new paint job. Use a heavy-bladed scraper that will not bend or dig into the surface as a lighter scraper will. Scrape away as much of the old paint as possible, remembering that the goal is not a paint-free surface, but a stable, smooth surface that will hold paint well.
Apply heat to especially stubborn or built-up areas with runs and drips in the original finish. Use a commercial heat gun rather than a torch, which can be dangerous. Work in a small area and keep the gun moving slowly over the built-up area. Wear gloves to protect your hands. Follow behind the heat with a heavy scraper. Remove any paint that becomes soft under the heat.
Sand the walls to complete the removal. A heavy belt sander is fastest and will cover the most area in the shortest time. Sand areas that are rough and will cause lines or build-up in your new paint. Be careful around any splintered areas to minimise damage.