This was a knob with a tired, mostly worn finish, that I just sanded, IIRC, with 220 and then probably 280 grit dry paper...color will depend on how bleached the walnut under that top coat is.
On this one, from a 356, I think I wet (water and paper towel, or maybe I just licked it...every wood guy has done it
..) it to see what the color would look like..It didn't need any stain, so...I finished sanding on a dowel rod, as shown. (I masked the logo and bottom in this case), and just started (as Erik just said) spraying light coats (probably 4 or 5) of Minwax satin polyurethane, following their recommended dry times between coats) while rotating the dowel. Always spray the can upside down until clear air to ensure the nozzle stays clean after each application, of course. Rattle-cans have come a long way as to how well the spray nozzles patterns apply product...
I may have rubbed this knob with Meguire's headlight polish, Plastix, after a week or so of curing...gives a nice luster. Need a higher sheen?...use a hand buffer. This knob is Bjoe's on this forum now...if you need a witness.