pablohoney -
As fate would have it, our original ’O’ series used Danish Oil as a first coat to seal and warm the tone. That went out with the 1983 CS3.
Our sealer or shellac are clear, which in shellac land is called ’white’, unless you want some color to match your aged veneers, etc. Then there are many shellac shades available beginning with ’blond’, all the way to ’dark amber’ and ’orange’. Begin with the thinned clear wash coats like I described. Just wade in.
I advise against polyurethane as that chemical family has much larger molecules and different light refractive index that I disparaged above. Try for lacquer to get a ’Thiel’ look.