I am looking to buy a pressure washer to keep various outdoor surfaces clean. What is the maximum safe pressure the vinyl siding can withstand to prevent damaging surface etching? Also, will vinyl brittleness in below-freezing temperatures be a risk requiring a lower pressure or to avoid this method of cleaning?
Do NOT use a pressure washer on vinyl in below freezing temps. This is just asking to shatter the plastic, especially if it has had some exposure to the sun. That will make it more brittle. You don't need to clean the siding so often that you need to do this in the winter anyway. A hose has always been sufficient to clean siding for us, once a year in the summer.
Instead of the maximum pressure that won't damage the vinyl, find the lowest pressure that will clean it. Stay back from the siding a good distance and use a dispersing nozzle (one that fans out the water) to further reduce the pressure and to make the job go faster. Be sure to spray from above and in the direction that the siding overlaps so that you don't force water up or in through the joints.