Our front doors are solid oak with very nice glass panels. However, the oak is now partially greying, splintering, and much of the finish is gone. What is the best way to refinish them. Chemical or sanding? After preparation should a clear stain be applied? What is the best finishing coat? A spar varnish? Oil or water based? Or are there special polyurethane finishes that have greater UV protection? Googleing yeilds a variety of answers. Does anyone have any actual experience and advice?
Brand name is unimportant here. All the oxalic acid bleach products I’ve used are simply crystalline oxalic acid; no additional additives.
The BEST way to finish exterior wood is with high solids, light colour PAINT, with opaque stains coming in a distant second. Of course you don't like that answer, it obscures your beautiful oak. The reason it's best though, paint takes the brunt of UV degradation, and a quality paint is fairly easily maintained. So, you prefer woodgrain. Yes, there are "spar varnishes" that have UV resistant formulations. The thing about "varnish" to an old feller like myself is that in the past decade virtually all have become urethanes, and form coats unlike older blends of what are quite similar ingredients. The downside to urethanes, they do not form monolithic coats but distinct layers, and when the coat breaks, there is a tendency to peel. The peel is resistant to feathering, touch-ups won't blend, meaning the entire finish comes off for good adhesion and appearance. Some stains-using the word as applied to exterior home finishes- have film forming characteristics that can be scuff sanded and re-coated, and I suspect have an older varnish component along with pigment. The apparent trade-off, ease of maintenance vs. frequency of finish replacement, isn't so clear. IME, Spar polyurethanes (marine and the like) will give two years service before looking crappy and developing surface flaws. That is with sun exposure of ~6 hours daily, no protection from eave, storm door, etc. With older varnishes, they become somewhat "chalky" in one year's time with the same exposures. I prefer an hour or two of scuff sanding and wiping on a new coat, in situ, annually, to the usually two day ordeal of pulling doors, stripping/sanding/re-coating and waiting to dry that is polyurethane. If we could flag down a chemist, an explanation of oils and polymerising would be of further help in your selection.
I’ve had good success with oiling treatments providing that the wood is protected for at least some of the time. (shade or out of he rain) I started adding oil after the first rough sand at 80 grit, and added layers progressively at each new grit up to 600. For all those I used teak oil which is preservative. For the last layer I used linseed oil, and finished with orange oil. If you can be bothered to wipe the door down once a week or so with a light coat of orange oil, you’ll maintain your natural wood color for a long while, though some silvering will still occur.