It all depends what condition the original finish was in when it was painted, what products where used on the original finish, and what preparation was done before it was painted.
If you're lucky, the finish was in good to fair condition, various waxes & polishes were used on it over the years, & minimal to no preparation was done before painting.
I've stripped the paint off quite a few original finishes on a variety of antique furniture. Some have required not much more than a wax, while others had to be evened out with a solvent or coat or 2 of shellac.
You've really got to experiment to find what works best for a given piece of furniture, and the end result can vary greatly.
I've found paints that desolve in ammonia, petrol, kero, & some general household cleaning products, but leave the original finish untouched. If I run out of things to try, then I resort to scraping the paint off.
If the surface wasn't well sanded before being painted, you'll normally find that a barrier has formed on the original finish from dirt, dust, oils & waxes etc, so the paint more or less sits on the surface. If this is the case, you can use a knife or dull single edged razon blade to lift the paint from the finish. Sometimes once you get an edge up, you can peel the paint off in big sheets with your hands.
If however the finish was sanded down properly, then the paint will usually adhere really well, and there's nothing you can do except strip everything & start a fresh. And if a pretty good finish has a few gouges it will stick to them, or soak into bare timber if they're deep enough.
The video below shows how simple it
can sometimes be, although the finish underneath isn't in very good condition, but at least I'll be able to preserve the original stain & colour to some extent, and it should come up ok with a couple coats of shellac.
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http://www.youtube.com/watch?v=viotfPVqZ3U[/youtube]