A pressure washer is a great way to make paint prep work quicker and easier, but there are some important things to remember.
First, always make sure you wear safety glasses around these things, and be sure the water is connected and turned on before you start the motor.
Most units offer four different tips to create different size fan patterns. Skip the zero-degree tip, you can do more harm than good with it. For most jobs, twenty degrees or so is just about right.
However, you still have to be careful not to get too close to the surface. Work your way in gradually to find the most effective cleaning distance, without creating any damage. This will take care of the dirt and grime, but remember pressure washing is still no substitute for scraping peeling paint.