Just say no to poly. Someone else may want to refinish again in 100 years. Stick with finishes that can be removed like shellac or lacquer. Amber shellac would be my first choice unless it's in an area that gets a lot of abuse of moisture.

You don't say what the original finish is on your 100 year old trim. Was it stained and then varnished?

I believe it just has shellac as it gets soft during the summer months. I think the color is a tribute to age and dirt. Some of the doors are quite alligatored.

How long would an amber shellac take to darken? Can I stain it first so the color mathches now, or is this a bad idea? Is there anything I should know about applying it? Why is poly so popular?

Sorry for so many more questions.

Polyurethane is popular because the film is durable and easily cleaned.

Test for shellac on existing old finish: If denatured alcohol removes it, it was/is shellac.
_some_ polys very much resent being applied over even a trace of shellac. The poly will turn white in a few days after applying it. That's a very good reason not to use it. Shellac is the fastest drying varnish out there. You can apply three or four coats in a day. You can mix your own shellac fresh from flakes by dissolving it in alcohol (or better still Behkol, proprietary shellac solvent) as fresh shellac dries the fastest and hardest. For my old house woodwork I use two coats of "garnet" shellac ( a deep ruddy color) followed by several coats of blond or ultra blond. After the finish has cured, I apply paste wax (Johnson's, Minwax finishing wax, etc) with 4/0 steel wool, and buff with a woolen cloth. The patina thus achieved is rarely surpassed.(except by french polishing - and you can do that for the doors where it shows to best advantage) It's smooth a silk, easy to dust, reversible, none of which can be said for a poly varnish, which unavoidably dries with dust specks, etc. that really cannot be rubbed out.
Just my two cents, as a life-long historic preservationist, old-house carpenter, and finish dude.
Casey

like goylesgirl, i too live in a 100 year old house w/ beautiful unpainted trim finished w/ shellac. sombreuil, your instructions are the clearest i have read to date, but i still have a few questions. do i strip all the old shellac first (it's bubbly and patchy) before i refinish? what would i use for that? denatured alcohol? formbys? is there any sanding involved? any special tools or brushes or brands? any advice would be appreciated.

My house had all the trim but the doors painted when I moved in. I have one room finished, the MBR, which I did over the summer. The other rooms are in various states of "Genteel Decay". You're lucky to just need to renew your finish. If there are water-stained window sills, you may want to do some heavy sanding before refinishing- it's really nice when they are smooth. As for a pre-shellacking cleaning process, try very fine (3/0) steel wool with windex or Fantastic brand cleaners. they will remove lots of dirt and any waxy yello buildup. (purists may insist that steel wool tends to enable tiny rust stains as the fibers get embedded in the wet wood. They may use bronze wool) The bubbling will probably be broken down during this cleaning. If you actually have "alligatoring" (bubbling squared) you will need to sand with 320 abrasive. The gold-colored sheets from Lowe's or HD do well. The problem is that the subsequent coats of fresh shellac will exacerbate the alligatoring, and this sanding will have to be repeated for a perfect finish. Mostly those of us antique-house-dwellers are used to compromise, and often are satisfied with less than perfection. The whole lot can be stripped, of course. I use the cheapest methylene-chloride type. Formby's is terribly over-priced for this application. I used a light-duty liquid stripper by Parks for the shellac. And many rolls of paper towels. And a charcoal-filtered respirator. And keep a fire extinguisher at hand. Afetr stripping sand with 120, 150, and 220, then shellac. try Rockler's, Highland Hardware, or Woodworker's Warehouse for shellac flakes and solvent. Use a 2"w. natural brush with long, soft, white bristles.
Casey

exactly what i needed. thanks. i'm working on my son's room right now and should be ready to do some refinishing in a month or two. and yes i have several water-stained windows. genteel decay? you ain't a kidding.