Always use the "fan" tip. This cleans in a larger swath and also is not as damaging for instance as a single jet.
regardless of psi's I always start about 1 foot from my work and move in and out as the work requires. Moving in and out actually tweks your psi and avoids the damage. Never apply too much presssure or stay in an area too long where there is a seam. Also do not shoot up into a crack or seam. You do not want water behind the scenes.
What allot of people do not consider when preping a project for painting: make sure the surface is 110% dry before you paint. Power wash one day and let the surface sit for a day if the temp is not high or you have humid nights.