i've bleached cedar with a hudson type sprayer loaded with a bleach
solution (don't recall the recipe, but someone will probably post) and
a garden hose. spray on the bleach, let is sit a while, and rinse
thoroughly with the hose. don't really think you'd want to take a
pressure washer to a cedar garage door. the black might be hard to get
off. you might want to invest in an orbital sander if you don't have
one and give them a good sanding.