Simple, finish it and play it like &%*# for 70 years
For waterbased finishes there appears to be two front runners right now, KTM9 by grafted coatings and another from Target coatings, I'm not sure which of the target finishes is their best. I have been using KTM9 for the last several instruments with good results but I am now experimenting with an oil varnish that is looking promising. As for making it look old, you can add toners to the finish to darken and give the appearance of sun darkened old finish but how they really compare, I can't say. I don't try myself because I figure they'll get there someday.