I've seen some demo's on faux graining using a base color and then using a "strie technique" (dragging) with a darker color adding the grain effect. Seems to me that some of the old techniques used a big feather after laying down a straight pattern of darker grain to add detail, or even did it line by line using small detail brushes? House painters use the technique a lot on the inner side of metal exterior doors to blend in with the trim. The main thing is to not over work the paint - you have to lay down the detail, get it right in a pass or two and let it dry. There's lots of information out there, google "woodgrain painting technique" or "faux woodgrain painting" and work past the kits for sale to find instructions and tips.