The short answer: with enough fairy dust, anything is possible ;-)
The long answer ...
When we're talking about a floor that's pre-war in an apartment building, we're typically speaking about a floor that's been down since the 1920's or 1930's. In a townhouse or carriage house, if the floors are original we could be talking a floor that was laid in the late 1800's. The thickness of the wood is *probably* 3/4" although parquet can often be considerably thinner and possibly not be locked into the neighboring pieces with a tongue and groove. To keep things a little simpler, let's assume we're talking about a 3/4" thick piece of tongue and groove wood flooring, meaning the pieces look like the diagram below.
Most of the time, the deciding factor on whether or not you can refinish is determined by how much of the wear layer - part of the floor above the middle of the groove - is remaining. The piece of wood in the picture above is what your wood floor would look like when it's new. The wear layer can usually take about 4-5 sandings before it couldn't be sanded and refinished again. This is a diagram of what the the profile of the floor in your pre-war apartment probably looks like:
Your next question is probably "so how do I know if my floor is too thin to sand?". This is when I tell you gauging the thickness of a wood floor is more art than science and that a mere mortal like yourself could never understand the whims of the wood floor gods. Only partially true.
This is what I look for when trying to gauge whether a floor is too thin in a pre-war NYC apartment with the original floors:
- Cracking at the edges of the boards. When the wear layer is almost completely sanded off, you'll start to see cracking along the edges of the boards. Being the ever-inquisitive consumer that you are, you might be wondering "why?" ... or maybe you're, but I'll tell you anyway. You get the cracking along the edges because every wood floor that has a non-concrete sub-floor is going to have some give to it. As the floor gets thinner, the thin top edge of the wear layer overlapping the tongue will be more susceptible to cracking. If you look closely, you'll probably notice that you've probably got more of this cracking - and possibly some splintering of the boards - in the areas that get a lot of foot traffic, like by the entry door to the apartment or in doorways between rooms.
- You'll see nail heads. If you're seeing a stray nail head - no type of pattern - then your pre-war wood floor is almost definitely too thin to sand and refinish. Take a look at the diagram above to see how/why the nail head sticks through when the floor gets thin.
- Noise or creaking in the floor. This is a little more subjective and actually is more art than science ;-)
This isn't a one-size-fits-all type post for all pre war wood floors, but it should set you down the right path to determining what you can do with the floor.