The stock seemed to be in average condition. I decided to completely strip the stock, stain it and finish with polyurethane. Of course, there are many other ways to refinish a Mosin stick, but this method seems to be very easy and produce good results. I didn't remove the barrel band springs, cross bolt / recoil bolt or sling escutcheons. I just stained and polyurethaned right over them.
Original condition (alternate view).
The stock was covered with a thick layer of old shellac. Shellac can be repaired fairly easily, but I wanted a tougher finish. I started with 90% rubbing alcohol and green scrubbing pads. It was removing the shellac, but after 20 minutes of scrubbing there was still quite a bit of shellac remaining. I ran to the hardware store and found some Zinsser Furniture Refinisher. The can says it "Gently dissolves old finishes - preserves woods original color and character." It worked 10 times faster than rubbing alcohol and did not damage the wood, but the fumes were terrible! According to the MSDS it contains acetone, methanol and toluene. If you use any of these solvents, I strongly recommend wearing gloves and stripping the stock outside.
After stripping the shellac and de-greasing.
The wood was oily after stripping off the shellac. I suspect the cosmoline had saturated deep into the wood. I sprayed it down with some commercial de-greaser, let it soak for a minute then rinsed with rubbing alcohol. I repeatedly de-greased and rinsed a few times. That seemed to remove most of the cosmoline without harming the wood. You can also let the stock sit in the hot sun and wipe off the cosmoline as it seeps out every half an hour. This method may take several days.
But now the wood seemed too dark and dingy. I mixed some OxiClean with water (it wouldn't dissolve in rubbing alcohol) and poured it on the stock. After letting it sit for 20 minutes I rinsed it off. After repeating a couple times the wood seemed to lighten a bit, but it didn't look like new, unfinished wood. Maybe next time I'll try some liquid bleach.
After bleaching with OxiClean.
This is the time to repair any dents in the stock. They can be steamed out with a wet cloth and an iron. The process causes the wood fibers to swell up, hopefully returning to their original shape and resulting in a smooth surface. First, apply some water to the dent and let it soak in for a while. Apply more water until the dent is saturated. Next wet a cloth or towel. Lay it on the dent and use a hot clothes iron to steam the wood. Hopefully the dent will smooth out. You might have to steam it several times. Gouges can be filled with wood filler.
It's usually easier to achieve a smooth, gloss finish if you fill the pores. Mosin stocks are usually made of birch, which has fairly small pores. Filling the pores probably won't do much good. I never bother with it, but if you want to invest some time doing it now, you might save a little time later sanding the finish smooth.
After letting the stock dry completely I sanded with 320 grit to knock down the grain that was raised with the OxiClean bleaching. I cleaned off the sanding dust, hung it and applied a thick coat of Varathane light cherry stain. I waited about 5 minutes and hit it again. After about 15 minutes I wiped off the excess. The wood didn't seem to absorb much stain. The new color came out very close to the original color (before any work was started on the stock).
I decided to finish with polyurethane. Some people may feel that shellac is more appropriate for a Mosin, but polyurethane is much more resistant to solvents, water, abrasion and pretty much everything else. You can apply a sealer to hold in the cosmoline, but it doesn't seem to be necessary. Update... It's been 2 years and the polyurethane looks fine (no separation, peeling, cosmoline seeping or anything).
You'll need to choose between a gloss or satin finish. If the stock has a lot of dents and scratches a gloss finish will highlight every flaw. In this case a satin finish is probably a better option. Scuffs and marks are more apparent on gloss finishes. I recommend a satin finish for Mosins. I think it just suites the character of the rifle better. You can always change your mind and re-finish.
To prepare for finishing, sand the stock lightly with fine or extra-fine grit sandpaper. We're trying to get the surface as smooth as possible, but try not to sand too much and change the shape of the stock. Your stock probably has some dents that aren't going to sand out - don't worry about it. Remove all the sanding dust with either a vacuum, a cloth dampened with mineral spirits or a tack cloth. Don't use water or it'll raise the grain and you'll have to sand it down again.
You should follow the directions for whatever finish you use. I apply a very thin coat of polyurethane on every surface, inside and out. This seals the wood to protect from water, oil, etc. I apply the poly using a paper towel folded to about 1" x 3". I've found sponge brushes tend to leave little bubbles in the finish. A good quality bristle paint brush works well, but they cost quite a bit and can be a pain to clean. A paper towel is fast, cheap and easy - and actually leaves a decent finish. Let it dry for at least 4 hours between coats (drying time may vary).
Here's my process for a gloss finish: Apply four or five coats of polyurethane. It'll take a couple days. Polyurethane gets sticky as it dries, and it dries slowly so let it dry in a dust-free location. Otherwise you'll get dust "nibs" in the finish. After the 4th or 5th coat let it dry for an additional 12 hours to make sure it's completely dry. Then sand it lightly with 400 grit wet/dry sandpaper. Use plenty of water as you sand to prevent the sandpaper from loading up. Be very careful to use light pressure so you don't sand through the polyurethane (if you do, just apply a little stain to the spot and don't tell anyone about it). Remove the sanding dust with damp paper towels, then dry it with paper towels. Apply one or two more thin coats of polyurethane. Now gently sand with 1000 grit wet/dry sandpaper and water until the surface is completely smooth. Clean the sanding dust, dry it and apply a final thin coat of polyurethane. This should result in a very smooth and beautiful finish.
Completed gloss finish.
For a satin finish: Apply the poly until you have a few coats. You may sand for a little smoother surface, but it's not necessary.
You can stain with clothing dye (Rit is the most common) mixed with alcohol. Dye doesn't cover and hide the grain like stain does. It usually won't come up and mix with the finish as much as stain, resulting in a clearer finish. It's cheap and can be mixed to produce any color you can imagine. It can be more difficult to apply evenly. Be sure to practice on some scrap wood.
Watco danish oil can be found with stain already added. It's very easy to apply, just wipe on, let it sit and wipe off the excess. A couple coats is all you need and it results in a natural looking finish. Unfortunately, it only provides minimal protection.
The shellac finish can be restored. Sometimes all you need to do is clean the finish. If cleaning isn't enough then you can wipe down the surface with alcohol to partially dissolve it and polish it to a nice, new condition. If that doesn't work you can apply a new layer of shellac. I won't go into the details of how to do all of that because it could turn into it's own guide. There's some good instructions on the internet.
You can use many other finishes, but keep in mind the stock will likely get wet from washing out the barrel or maybe rain, it'll be exposed to various cleaning solvents and it'll probably get a little beat up through normal use - so choose your finish appropriately.