I'm curious too...what chemicals do you use? I recently did a small car port job that had several bad oil stains that were very stubborn. I used a 3700PSI 13 HP Honda cold water, 20 inch surface cleaner,degreaser and bleach on that job and it still didn't meet my satisfaction. I had to use a concrete cleaner (bought @ LOWES) and a stiff brissel deck brush to complete the job. Still not to my satisfaction but it passed inspection by the housing office.....Thank God!!!!! Has anyone ever used muriatic acid before?