So I have white travertine coping and an entire water wall. Over the last year it has gotten a little dirty. I used a 2300psi electric power washer today and it made it slightly cleaner, but u can still see ‘dirty areas’ especially in the middle where water seems to and eventually dry. Any suggestions. Stronger power washer, put dish soap or a travertine cleaner in the power washer? Any suggestions greatly appreciated!!!