I assume a pressure-washer might help you. My Father used one from Sears on his pressure-treated wooden deck and made it go from OLD BROWN to just like new color, so ………… try, try and try, maybe you will be happy, or ……… get a sand-blaster/bead-blaster (poorman’z version is air-compressor or tire pump ?) and use walnut shells or ??? ask expert at ? motorcycle shop or a nice OLD farmer. I like to leach helpFULL info from expert’z. My Father KNOW’z SOOO much ……… I quit. He told the VIP Auto Manager to call the Cop’z because he refused to pay for a wheel-bearing adjustment when he went in for a tire alignment …… because he KNEW from experience that the FIRST thing a proper mechanic does when doing a wheel alignment iz to check and adjust the wheel bearing. He think’z NEW ball-bearing’z come TOOO loose.