Tough to do much easily now. Scrape off the loose stuff. Use a stripper for the entire side(orange colored 'green' stripper is fine). Strip the remainder. wash with damp rags(see stripper instructions). and dry completely.

Sand lightly to smooth the surfaces(no more than 150 grit).

Stain desired color. Be aware the stain color will fade some, depending on the final finish.

If the door were mine, I would finish with clear paint. Have done that before. Sherwin Williams(the brand I use) A100 Ultra deep tint base with no additives---no tint at all. It is a milky white when applied and dries completely clear. I have pictures if you want of walnut finished with the base.

That finish(2 coats) will protect the door wood and color more than anything else. Because it is paint, not varnish/spar varnish, or poly. Paint blocks the harmful UV rays and protects from weather.

You're going to want to sand back to bare wood and start over again (the finish that hasn't peeled off yet will soon). The finish used was a pretty clear failure, but clear finish on wood exposed to the weather is always going to require constant upkeep. You should plan on refinishing every other year.

Look at products intended for a marine environment, like a marine spar varnish. Wooden boat builders now seem to prefer a clear penetrating epoxy sealer top-coated with a two-part catalyzed polyurethane. Ordinary big box store polyurethane is not intended for outdoor use. You'd have to order on line or go to a place like West Marine to get those finishes.

Paint offers far better protection than any clear finish, sorry to say. Stain will neither hurt nor improve your results, unless it interferes with the bonding of the finish coats.

From the weathering of the bare wood, it looks like it has been going on for two years. This is 6 months? This varnish was applied in Aug 2015?

Setting that aside, it's going to need full prep, sanding away all the surviving finish and gray weathered bare wood, down to at least 150 grit. You should probably use a dark gel stain like brown mahogany, (to disguise the possible water stains which may not sand out) and then four coats of spar varnish. If this is the kind of weather the door takes in six months, you will need to recoat the door every year as maintenance.

Casey

I just stopped by that cross I mentioned and took some pictures, after it has been in place for a year. Open to the weather, on a NE corner of an intersection, facing south and about 20 feet higher than the streets.

The first picture is after painting initially.

This picture is of the cross on the 19th of February, a year later.

As you can see, there is no graying/degradation of the wood after a year of exposure.