Dick Eastman had an incredible blog post called "Controversy: How to Read Unreadable Tombstones". It's an amazing video and it certainly produces results, but I would agree with Dick that I wouldn't want to personally use it or encourage it's use until I heard from qualified experts as to the long term effects of using flour on tombstones. Some may argue that the stones are already deteriorating beyond functionality. I would caution that before you do anything to a gravestone that you first receive permission by the cemetery.
Until next time have fun tending those roots! (Oh, and don't get behind on your reader posts...so not a good sign!)