I'm not familiar with Cetol, but after researching it I think that your idea of how to finish the door is a good one. be sure you seal the bottom and top edges well and it should be good for many years. I usually use a marine or spar varnish and have achieved good result with them. Cetol possibly costs more than what I use, but I would bet it is a good product since it is made by Sikkens.

I just used Cetol Door and Window on a fiberglas door and a pine Andersen window. It looks good on the door, but the window just *glows*! Since I just did the door a few weeks ago and just put the 3rd coat on the window this AM I can't tell you how it will weather, but it was highly recommended by Faron who usually posts on Home Decorating and Paint forums.

Now Sikkens doesn't recommend using just the clear Cetol on areas with high UV exposure, I don't know if they would say 3 coats of clear over your stain is fine or would talk you into using 1 or more coats of one of their tinted Cetol finishes. Tech Support was very helpful with instructions for finishing the fiberglas door (not on their website). Why don't you give them a call?

The pine window is vinyl clad, I just used the Cetol on the garage side since it's exposed to temp swings, but not too much UV and no precipitation, so I felt the clear was fine (since I had it left over from the door). My door is finished with a very dark thick Zar stain, so I didn't want to put a tinted topcoat on. It's also under a porch.

Faron just suggested keeping ahead of any damage, so depending on UV exposure (esp. at the bottom of the door), you may have to wash the door and put another coat on every 3 years or so.