just be sure to keep it wet the whole time. keep whiping it with a rag or squirting it with a spray bottle. if its the same water spots as what comes out of sprinkler systems onto car windows then a razorblade is going to slide right over it. and you will risk digging the corners of the blade into the glass which is much harder than steel wool. steel wool isnt that hard.

the last guy i did this for gave me a $20 tip for showing him how to get it off his windows saying he was about to have to replace all the windows on his car. he said he tried razorblades and every chemical he could find around his house and nothing worked. i did the whole driverside window of his thunderbird in 5 mins with some steel wool and water. keep me updated.