I've just salvaged a house worth of 90 year old Oak trim, decorative woodwork, a couple of doors and a built-in buffet/China cabinet from a Chicago 2-Flat. It's all got about 8 or so layers of paint/Finish on it. My desire is to finish it with a clear (possibly stained) finish and replace all the fake-wood trim that currently infests my home.

With a heat gun, the 7 layers of paint comes off pretty easily and leaves behind the wood and a layer or two of what I assume is the original varnish/finish. My questions are these:

1) Do I have to remove this also and go down to the bare wood, or can I can I clean it up and leave it on?

2) If I leave it on, what should I use over it and how should I prep the surface?

3) If I need to strip that last layer off, what chemical will most easily and cleanly remove it?