Direct answer
What should you know first?
Pressure washing removes surface dirt, algae, traffic film, and much of the winter residue from sound concrete. Oil, rust, leaf tannins, and deeply absorbed stains may require targeted treatment and may lighten without disappearing completely. Even cleaning depends on equipment, overlapping passes, drainage, and the condition of the slab.
Identify the buildup before choosing treatment
Green growth in a shaded area, gray traffic film, white salt residue, orange rust, dark oil, and leaf staining are not the same problem. The likely source sets the treatment and the expectation.
Why even passes matter
A surface cleaner can produce a more uniform result across broad slabs than random wand marks. Edges, steps, and tight areas still need detail work. Rinsing and managing loosened debris are part of the finish, not an afterthought.
Know the limits of older concrete
Scaling, exposed aggregate, cracks, and deeply absorbed contamination may remain visible after cleaning. More pressure is not always the answer; it can make weak concrete worse. A professional estimate should point out those limits before work begins.