I'm by no means an expert, but I have used the borrowed one several times so I've developed a bit of a technique. I always use the fan, and start at a good distance away where it's not effective. Then I gradually get closer until the muck is coming off without damaging what's underneath. With the borrowed one (which I think was about 2500psi/2.3 GPM) I had to get close enough that the fan was 2-3" wide. My hope in getting a more powerful unit is that I can back off enough for a wider fan to be effective. Wouldn't it be nice to knock off a 4-6" wide swath at a time? Wishful thinking, though.
Last fall was my first haulout, so my first chance to enjoy the pleasure of getting under the boat to wash it.
There were a couple of times where I fumbled with the wand and it almost touched the bottom, but there was NO DAMAGE! Man, that WM Bottomshield paint must have been tough stuff! With 3000 psi I need to be even more careful because that might cause more damage. This time around I have Pettit Vivid, which is a slightly softer hybrid paint, so hopefully the slime will come off a little easier. We'll see.