Paint?

Keeping clear finished outdoor wood unchanging and looking like new is almost always a losing battle. You'll want a finish with lots of UV blockers and plan to refinish on a 12-24 month schedule, just like maintaining a wooden boat.

Or you can use paint. Clear paint. Most paint manufacturers make an ultra deep tint base that is actually paint with no color. (I use Sherwin Williams Ultra Deep Tint base). It is milky white when applied, but dries clear.

Here is a walnut cross I built and installed two years ago. Applied two coats of SW A100 Ultra Deep Tint base(cannot use primer), installed the cross in March of 2015, took this picture a year later, and recoated this April.

Full southern exposure. There is a little degrading on the top surface of the right cross piece due to moisture/sun. I simply hand sanded the entire surface of the cross and applied a coat of the paint.

This is what the paint looked like when first applied(before installation).