Regular oil varnish will not dissolve in any solvent, same goes for poly. Only lacquer or shellac will re-dissolve in their solvents. Try lacquer thinner to rule that out, then figure it's a drying varnish. (as opposed to "spirit varnish"- [shellac or lacquer]).
Casey

okay, now weRe gettin somewhere. thank you mongrel. turpentine didn't work either. i am bummed about it not being shellac, as amber shellac is the most dead on thing i've found for covering the scratches in my 1905 fir paneling and doors. but... on i go. i have some xylene i will try. if that doesn't dissolve it, nothing will. that is paint thinner. isn't that the same as lacquer thinner?

Paint thinner is a very mild solvent, it doesn't have any ability to cut into a finish. (unlike acetone, lacquer thinner, toluene, xylene, etc) IOW, you can wipe tar and gunk off your car's paint finish with paint thinner w/o harming the finish, which cannot be said for any of the other solvents I mentioned, which will do varying degrees of harm to paint. Paint thinner barely removes traces of label adhesive; it's only slightly more aggressive in doing so than baby oil.
Casey

Lacquer thinner will pretty much cut or dissolve any finish if left immersed. Try taking a picture of the flaking area and send that. If amber shellac matches best, you may have a "glaze" coat applied on the sill. This is a tinted clear finish to darken or even out blotchy wood. It's anybodies guess as to what the actual finish could be. Good luck.

The only finish that would likely (1) be found in a windowsill and (2) come up in flakes that you can feel and pick up is polyurethane/varnish... shellac and laquer wont flake up like that. but it does not matter so much what the old finish IS, if you can touch up the color with a DEWAXED shellac and topcoat with more durable modern finish, you're done..
I would clean the whole sill area. sand lightly 220 or 300 grit, tack away dust. touch up defects. make sure its zinnser sealcoat DEWAXED. you can tint this to match with Homestrad finishings TRANSTINT dyes. Do NOT use a shellac that has WAX in it in a windowsill, ever. I would coat the whole sill edge to edge, over your repairs with sealcoat thinned 50/50 with alcohol, then topcoat with a modern waterproof poly.
In windowsills where UV resistance is important I have had great success with maxtech waterbased urethane but minwax water poly may be more easily available to you . use at least 2 coats topcoat.
good luck and enjoy!
Reid

thanks. so Reid, i used Isopropyl alcohol. is that the right kind to dissolve shellac? i don't know if it has to be ethanol or what isopropyl acholol is.... xylene didn't dissolve the flakes either

i already covered the whole sill in amber shellac. it is the wrong shade, anyway. the whole casing and sill and windows are coming off soon to be sanded and stained and poly coated if i can't figure out what is on there.. i can try your dewaxed stuff on the casing. i would, of course, prefer to just lightly sand and re-varnish, rather than the whole hog tech. just described, esp the window sashes.

question: what is your "sealcoat" you refer to? thanks again

the "varnish" if that's what it is, is a satin sheen. and the color of the old growth fir is very close to honey oak.

"i used Isopropyl alcohol. is that the right kind to dissolve shellac?"

You need denatured alcohol, unless you want to spring for Everclear grain alcohol.

The color you are describing sounds very much like one of the less refined shellacs.

Shellac will also whiten if you put water on it (though older stuff can take some time).

The really big guns are things like methylene chloride (used in stripper) and MEK.
These can remove just about any finish if given enough time.

They do not 'dissolve' the finish like alcohol and shellac (or lacquer thinner and lacquer) but chemically attack it.
MEK can dissolve epoxy.

Paints and clear film finishes come in two forms. Reactive and evaporative.

Evaporative types are those that dry when the solvents evaporate leaving the suspended resins and pigments. These finishes can be re-dissolved with the same solvent used to make or reduce (thin) them.

Reactive finishes are those that dry when the solvents flash off and cure (harden) by a chemical reaction. The polymerizing or harding of the finish resins are activated by oxygen in the air or a chemical additive. These finishes will not re-dissolved using the same solvents used to reduce them with.

Evaporative finishes include:
NC lacquers
Acrylic lacquers
Shellac
Oils - mineral oil, paraffin oil, most vegetable oils

Reactive finishes include:
Oils - linseed oil, tung oil, danish oil, teak oil
Varnishes
Urethanes
Conversion varnishes
Epoxy paints
Pre and Post catalyzed lacquers
Polyesters

Some clues to ID a finish include testing with different solvents to see how they react, considering the age of the item and what was typically used during that time frame, sanding the finish and smelling it and there is a product called "Doctor Dan's Finish Indicator". You put a small drop on and see what color it becomes.

Steve

Here is a link that might be useful: Finishes for Furniture

thanks steve, alittle shocked to see linseed oil, tung oil and teak oil, something i am going to post about right now, on the list of REACTIVE finishes. but. i will try that link and getta bottle of Dr. Dans. thanks. if it works i will be wishing i had gotten some five years ago!