This is what will give the wooden stock longevity and enhance reliability.
The object is to seal moisture out of the stock. We will stabilize the wood by sealing it. The more moisture in the wood the more the wood will react to temperature change causing stress on the barreled action, thereby causing precision to suffer. Of coarse sealing the wood also helps protects it from the deteriorating effects of weather. There are other sealers but tung oil or modified tung oil is the usual choice for sealing a gun stock and filling its grain. The last rifle I finished I used a product called Waterlox Original Sealer/Finish . It is a tung oil product that was recommended to me and I do like it. I was not able to find it locally but the fine people at Waterlox quickly sent me a quart C.O.D. at my request. Brownells offer some other good tung oil products specifically for us.
It is important to realize that upon sealing the stock any moisture currently in it becomes locked into the wood. For this reason, I advise not applying a sealer while the ambient atmosphere is or has recently been high in humidity. Rainy days are not good here and, in fact, I would place the bare stock in a warm low humidity area for some time before applying a sealer. If too much moisture is allowed to be trapped inside the wood it could cause grain to open and disfigure the finish during periods the firearm is subjected to hot dry weather.
For me, sealing the wood is the easiest and one of the most enjoyable parts of finishing a stock. Now finally the basic color(s) and figure of the grain will be exposed. We do not have to worry about getting the sealer on smoothly or, except for non-wood parts permanently attached, wiping it off. We do want the sealer to be very thin and to apply it to all areas of exposed wood. This means that we will dilute the sealer so that it will penetrate deep into the wood and that we will apply sealer to all the stock including (if possible) areas that grip cap, butt plate, etc. attach to, barrel channel and action area and inside holes that sling studs reside. The entire stock must be sealed. Areas of glass bedding and factory bonded parts such as grip cap and forend tip are already sealed by the bonding.
Before sealing the wood you may know whether the stock is to be (glass) bedded . Although not essential because areas to be bedded are roughed before bedding, if the stock had not yet been sealed, I might elect to bed the rifle before sealing the barrel channel and action area, thereby allowing the bedding compound to soak into the grain. The rest of the stock should be sealed and filled before bedding. This keeps the excess epoxy that pushes out during the bedding process from soaking into unwanted parts of the stock thereby discoloring it. Even though it is wiped off as it is exposed it will still penetrate into the wood if no protective barrier is present. The epoxy bedding will seal the wood and no sealer should be applied to the surface of the finished bedding. Sealer contacting bedding should be wiped away. Do seal all inletting joining the bedding.
The stock should be clean of dust or whatever before applying the sealer. If you used steel wool on the stock, be sure no fibers were left behind. Those little buggers get in everything they can find. If you miss any now you'll find them after the final coat of finish has been applied.
To seal the wooden stock you will need mineral spirits, tung oil, a small container to mix the two and a small clean brush or lint free cloth. This part is easy and fun. Mix 4 parts mineral spirits to 1 part tung oil. A little more mineral spirits is not bad but more tung oil is. Seems too thin? You want it to be. Get that oil deep. You can apply another after this one dries. Replace the cap to the tung oil container soon as it thickens quickly and you will need it several times again for filling the grain . Saturate all the stock you can but only once. Do not put more on a little later but do check to see if any spots were missed. Periodically turn the stock for gravity to help deepen penetration on all sides. Let the stock cure at ambient room temperature in a dry ventilated place for at least one full day. Two days is better. Do not allow the fresh stock to be subjected to direct sun light or try to hurry the drying process by heating it. Enjoy the beauty of the wood that the oil has begun to bring out and inspect for areas that need more work (a missed sanding mark, whatever) and correct them before applying more sealer or finish. If, after the sealer has dried, you decide to do another coat keep it diluted as described above. The wet sanding procedures that follow will also help to seal the stock, however, the deep sealer coat is essential to stabilizing the wood.
You can extend the life of your oil by putting small clean gravel into its container in order to evacuate excess air.