Sand the outside of the door until the color is even and all the finish is gone. Sanding will take a long time and a lot of sandpaper---use a random orbit sander---NOT a belt sander!!!!!---but will not hurt the surface of the wood. Start with 100 grit and finish with 150 grit.

Apply an outdoor deck type stain if desired.

Now, use 200 grit sandpaper on a hand sanding block---buy one or simply use a flat piece of wood/plastic---and knock down the fuzzies left by the stain.

Using a really good brush---I prefer brushes made by Purdy and use a manmade bristle variety---apply the first of three coats of marine spar varnish with UV protection included or added. I prefer spar varnish from Sherwin Williams paint stores, but any name brand spar will work.

Let dry 24 hours. Lishtly hand sand , wipe with a rag dampened with thinner to remove all the sanding dust/grit, and apply the second coat.

Let dry 24 hours and repeat sanding and the third coat.

Now, from this point, all you need to do is inspect the finish monthly. Once you see checking/crazing----that is when tiny cracks start to appear-----simply sand with 150 paper on the ROS(random orbital sander) and apply one coat of spar varnish.

The first job should last a year or more. The only caveat is that the spar varnish will add an amber tint to the door color----most folks call that tint a warm amber glow and prize it for interior finishes.

This is an oft-asked question in another forum. You will have to remove the existing finish and damaged wood first.

Here is a link that might be useful: An innovative solution

Yup, that base paint is a good idea---if you can get the right stuff. If you try that advice, just be sure to test paint a piece of wood to assure yourself that it does dry clear.

And make sure all those mildewcides/fungicides are actually included, and it is an exterior base---not an interior base.

As for the spar varnish turning black/etc----I can show you several doors I have done that have several years of time with the spar varnish applied as I outlined---no such problems.

Even if you do a perfect refinish job using the best marine grade varnish, this will be an annual project.

I'd consider adding an awning to protect it from the UV.

I'm in a similar situator and plan on painting it as I already have too many maintainence items.

Just a thought. You didnt say how far up the bottom was faded. How about a brass kick plate. Ive seen some that were two foot tall.

Theres other metals too if you dont like brass. Or if thats too traditional you could have one made. Hammered copper. Cast aluminium.

I can understand not wanting to paint your door. Im thinking since the top stays okay maybe you could come up with something decorative to cover up the bottom.