I purchased the first consumer power washer on the market, the Karcher HD555, in the mid 80's, ordered it before it was available, and still use it today. It has frequently run non-stop for sometimes 7 hours at a shot,(
almost continuously). If I had a nickel for every time someone told me it would peel the paint right off a car I would be rich.
Ditto for other surface damage.
It has never happened, although I've been warned that I might polish the paint off a few cars from waxing, (facetiously). Some common sense is required.
The way to avoid excessive damage to a surface is to simply increase the distance from the spray tip to the surface. If you aren't sure, just start in an inconspicuous area until you become familiar with its power. I find you will need to hold it close to individual stains momentarily, then move back and continue on. Of course, if there are already weak points (cracks) in the gel coat, you want to keep your distance from them. Same with any seals and window edges, and decals, although I have never had a problem with a properly-applied decal or pinstripe. The angle from the surface that you hold it at can make a difference as well.
Power washing is a very environmentally-friendly cleaning method (if the electricity is generated cleanly), as it can be done for the most part without harsh chemicals, (pre-treat potential problem areas) and often you can siphon water from rain barrels or other non-tapwater sources (the lake, or your pool cover in the spring), if your machine has that capability.
Compare different units by multiplying the pressure by the volume output, and use that number to compare output, as a higher pressure will put out less volume for the same power input. The wands and nozzels add another variable to this, though.
Also know that it does not eliminate some old-fashioned brush scrubbing, though.
Make sure you drain it completely before freezing weather hits, or it's toast.
-Brian.
