Acid washing Saturnia

Outside in my walkway i have cross cut travertine. It has beautiful Colors (http://www.g640.net)
and (http://maplered.com) imperfections but only when its wet. I heard about acid washing. and (http://maplered.com) i
tried it. i think it helped but how long should i leave the acid on and (http://maplered.com)
should i seal the Stone (http://g603.net) after i get the desired "natural look?"