Dare to Prepare
Preparation of the stock is very important. Without proper preparation of the stock, the best finish will not be nearly as attractive and durable as it should be. Take care during all phases of the project and you will be repaid with a fine job that will have your hunting buddies drooling on your old faithful rifle or shotgun... and the new finish will shrug off their slobber without hesitation.
Naturally, when refinishing a gun stock, the old finish needs to be removed. Many folks attack their stocks with sandpaper to remove the old finish, and I can't stress enough that sandpaper is the WRONG thing to use for stripping the finish from a stock in most cases.
Sanding removes wood unnecessarily, may make for a wavy surface, and ruins positive checkering by removing the points that make checkering desirable in the first place. You must also use care when sanding areas where the wood meets metal - it's very easy to round edges that should never be rounded.
The use of a chemical stripper not only makes the job go faster, it allows you to do the job without damaging the wood stock that you are trying to preserve. While it may take some serious elbow grease to do the job even with chemicals, the end result beats the heck out of an over-sanded stock, and it almost always makes the job faster and easier to do.
To remove finish from checkering or other tight spots, use an old toothbrush (or similar brush) to work the stripper into the pattern and to remove the old finish. NEVER use a wire brush - it will scratch and gouge your stock, no matter whether the brush is made with steel or brass bristles.
Follow the manufacturer's instructions for the stripper you're using, including follow-up procedures for cleaning the stripper from the wood after the finish has been removed, and be sure to use gloves and whatever other safety products the manufacturer recommends.
Pressing Your Stock
After stripping the stock, iron it. No kidding! Take a hand towel or a similar rag and get it damp, then dig out mama's clothes iron and press that stock. Keep the damp cloth between the iron and the wood, move the iron around so you don't overdo it in any one spot, and be careful not to gouge the stock with the pointed end of the iron.
You can iron out just about any dent and most gouges in this way. This means you don't have to try to sand away a bunch of wood around dents and flaws and possibly lose checkering and/or end up with a low spot in your stock - instead, you just raise the dented wood right back up where it belongs.
In cases where the old finish is compatible with the new finish, some folks prefer to iron the stock before removing the old finish, and then carefully sand the old finish off using 220 grit paper. This allows the old finish to act as a filler, working the powdered finish into the gaps that exist in the grain of the wood. This may work well, but I have never tried it.
Starting to Sand
After stripping, damp ironing also raises the grain of the wood, so the stock won't be super-smooth when you're done. It will actually feel a little rough in your hands. Next, sand judiciously on sharp corners you might want to round off and do any other shaping you might want to do, with as fine a grade of sandpaper as you can get away with.
Always use a sanding block everywhere possible - sanding with your hands alone will cause low spots as the sandpaper will remove more wood from softer areas of the grain, and you'll end up with a wavy, ugly stock.
Now that the shape is the way you like it and the grain has been raised and dents removed, sand the stock as lightly as possible. Remove as little wood as you possibly can.
Never sand a portion of the wood that abuts something else (like a butt plate or receiver), unless the "something else" is installed (and then of course you have to be careful not to mar the receiver or whatever - one or more layers of masking tape can help protect adjacent surfaces). If you sand the wood say at the butt without the buttplate being on it, you can easily round the wood and when you're done you will have a sorry-looking stock, even if the finish itself is wonderful.