Painting over old exterior paint can be a difficult task, but there are several ways to go about it. If your home is built on concrete or has an unyielding surface, you may need to remove the existing paint and seal the surfaces before painting. This will help prevent moisture from penetrating through the new coat of paint and causing damage. If your home has wood siding or other non-tough surfaces, then you might be able to just apply a layer of primer first and then finish with a fresh coat of paint.