This may be rubbish, so take it with a pinch of salt, but I believe that a large number of medieval bows were painted with various ingredients. To them, the look of the bow was far less important than the resilience to weather conditions such as intense sunshine or rain, so they wouldn't have cared whether or not they could see the wood grain and so on. Lots of the Mary Rose bows were definitely painted, some with a black mixture I assume was soot mixed in with various oils and waxes.
I think, personally, if you're going to "go medieval" you should theoretically paint the entire thing with a soot and wax compound. However, to modern eyes that seems a shameful cover-up of what is usually a beautiful piece of craftsmanship, so maybe a slightly less genuine finish of beeswax as Adam said.
I've found that making up your own paste from pure beeswax, boiled linseed oil and some form of spirit works very well in all weathers, leaves a stunning rich patina and also dries to form a hard clear coat, unlike only using beeswax which can end up getting a tad mucky!