Old military guns in particular. If you've ever bought an old surplus rifle you'll know what I mean. Some aren't too bad, others are awful. Especially British and some U.S. rifles. Many stocks were oiled to help preserve them, and they soaked up oil from the metal. A lot of them soaked up so much that when the wood gets hot from either the sun or shooting, oil seeps out. And then, of course, there's Cosmoline.
Cosmoline is a grease that was designed for one thing: to protect metal from corrosion in long-term storage. And it works, very well. The bad thing about it? Getting it off. It is the stickiest stuff you've ever ruined your pants on, and the British in particular used a lot of it; the climate I would assume. When a rifle was sent back for service/storage they would dismantle it; anything needing work was repaired or replaced; then the bore was filled, the action & barrel greased thoroughly and replaced in the stock; and sometimes the wood was given a coat. When it was placed in the rack or crate, it could- did- sit there for years into decades, well protected from rust. Which means when you get it...
Did I mention that if it has sat long enough, the stuff can harden? Petrified Cosmoline is even worse to get out.
The metal's not that bad. Most any cleaning solvent, including hot water & soap(I've know of people taking one to a car wash and using the engine cleaner spray on it) will cut the grease off; my favorite is either low-odor mineral spirits or a can of carburetor cleaner. Either will cut the stuff and leave no residue behind(yes, use rubber gloves and ventilation). However, some of this stuff can harm the wood. There's several ways to clean this off, degreasers and such. My favorite, which does take some time, is the kitty-litter method.
Strip the stock to wood only and wipe off all the surface crud. Get some heavy plastic and make a bag wide enough to put the stock in and about a foot longer, make sure you seal it well enough to hold some weight. Put the stock in, and then fill the bag with either cheap unscented kitty litter or oil absorbent. Close the bag and lay it out in the sun all day. Next day, put it back out on the other side. Depending on temperature and how much oil/grease the thing had soaked up, a week to ten days should do it. The heat will cause the oil/grease to bleed to the surface and the litter will soak it up. When done take it out and go over it with a brush to make sure you get all the dust out of the holes and inletting. I've seen an old stock come out of this treatment looking almost like new wood. Don't throw the absorbent away, save it for when you spill oil on the driveway and such.
The other way to do this is much faster, but only works if the stock is short enough to fit in the oven. Prepare the wood as before. Take a roll of cheap paper towels and stuff the receiver area, barrel channel, etc. with them, then wrap the entire stock with at least two layers. Wrap the whole thing in aluminum foil, stick it in the oven and turn it to 'low'. Please make sure the stock is not touching the coil or right on the bottom above the burner, whichever. Leave it there for at least an hour, pull it out/yes, it will be rather warm/ and unwrap. Throw away the paper, which will be soaked, repack & wrap the stock and put it back in. It does the same thing as the litter treatment, it just does it faster. For smaller pieces you could probably either use a metal pan full of litter, or a length of metal tubing; put the wood in, cover with litter, seal the end of the tube a bake.
Another thing that works well on smaller pieces is to make a mix of one part ammonia to three parts water, get a soft brush and start brushing that over the piece. My understanding is the ammonia converts the oil/grease to a form of soap which the water can wash away. This works well on pistol handgrips, handguards & such. You may not want to use this on pieces that are very thin, as it can cause warping. The piece will then need to dry thoroughly before you can refinish it. I can attest that this will get crud out of a piece of wood that looks clean, used on an oily pistol grip or handguard it can be amazing how much stuff winds up in the bucket.
Theres a forum here that specializes in this subject(their home site has lots of interesting forums), there's a lot there on both cleaning and refinishing. Some of their methods I've never tried, spraying oven cleaner on a stock for instance; that sounds a bit, oh, harsh? If something's in bad enough shape it might be something to try. I keep using the kitty litter method because
A. I'm lazy
B. Once it's set up and heating, I can do something else
C. If there are, or you suspect there may be, markings of interest on the wood, this method will not damage the
surface as some others might. I have to stress might; people on the Milsurp forums swear by some of the other methods. You makes your choice and takes your chances.
Finishing up depends on whether you're trying to keep the finish historically accurate or just get a good-looking finish that will protect the wood. You can use boiled linseed oil, you can use shellac, you can use Birchwood Casey's gunstock finish, there are lots of possibilities. Do some research and give something a try.
A lot of these are worth cleaning not only to make it neater to handle & shoot, but- rather amazingly- some of these old rifles have absolutely beautiful wood; when you get all the old crud off the surface it's sometimes amazing what you find. And it's always nicer to shoot something that isn't depositing forty-year-old oil on your face.
Update: ref the kitty litter method, if you'll be cleaning a number of stocks, you might want to make a sturdier container. I recently bought a 5' section of 5" diameter stovepipe for one. Cut it to 50" long, then mashed it into an oval cross-section, then made two plugs out of 5/8" plywood. One's screwed and caulked into one end, the other uses two screws to hold it in place. I also drilled a hole in the removable plug and set an eyebolt in it to make it easier to pull out. Then I painted the whole thing flat black. To use, pour a couple of inches of litter in, set the stock in, then fill it up, insert the plug and lock in place, then lay it in the sun.