To finish the 4 string fretless bass I've been working on recently, I chose to use a home made varnish/oil blend. It consists of 1/3 polyurethane based varnish, 1/3 boiled linseed oil, 1/3 white spirit (aka mineral spirits). The oil content brings out the depth and colour of the Walnut, while the varnish adds a bit more protection, compared to using oil alone.
To apply the finish, I fold a lint free cloth into a small pad and apply a liberal coat, working it in to the surface a little (some people like to sand with the finish to fill the grain, but I prefer the more 'natural' look of the open grain). It's left to soak into the wood for few minutes, and then the excess is wiped off.
It's left for around 4 hours, to ensure it's fully dried before the next coat is applied. I apply around 8 coats in total. The end result is a smooth satin finish, which looks and feels great.
To finish the neck I used a slightly different method. I thinned down some varnish (without the boiled linseed oil added), and wiped on several thin coats, sanding in between coats and after the final coat with Mirka Abralon pads, to give a silky smooth surface.
Once the finish has had time to cure, it's time to assemble the neck and hardware.. I'll have some finished pictures of this bass in the gallery in a few a weeks time.