Applying finish to the outside of the wood is not necessary but can increase sheen, depth and add some extra protection to the wood surface. It is much easier to remove future damage without a surface finish since all such finish will have to be removed in order to sand out scratches and dings. With only a classic 'in the wood' finish (the kind you have now if you've gotten this far) all you have to do is refill the grain where the repair was made. If you want a surface finish the stock should be sealed and the grain filled before proceeding. Do not apply surface finish to the barrel channel, action area or places other components attach to.
There are different oils, and finishes other than oil, to choose from. The most commonly used on gun stocks seem to be modified linseed oil (lin-speed), tru-oil (has some linseed in it), tung oil and products containing oils and polyurethane . Some are sprayed on, others are hand rubbed.
If you are one of those that can spray paint without causing even the tiniest run a spray on finish should turn out great for you. You can build the coats up pretty thick giving a deep polyurethane finish. The usual method for non-factory and custom stock makers is hand rubbing the finish on. Regardless of the method chosen, first check closely for any overlooked sanding marks or unfilled grain before applying finish on top of the wood. The wood will not be accessible once you begin and all surface finish will have to be removed in order to correct any missed imperfections.
If you spray the finish on be careful not to allow dust or lint to contact the stock during or after applying finish. You will not be able to remove them wet without leaving a mark behind. The stock must be completely clean of anything foreign that will be trapped inside the finish. The higher the gloss the more fibers, dust, whatever will stand out. The only way I know of to get them out is to take the stock down to the wood surface and start over. You should already know how to properly spray a finish if this is what you want to do. Other than to remind you that the last coat should be dry before the next is applied and the number of coats is whatever your heart desires, I will move on to hand rubbed finishing. Hand Rubbed Oil Finish Here you will not have to concern yourself about dust embedding into the finish. Each coat will be applied so thin that anything foreign will be wiped off. However, start with a clean stock and be just as sure as before to allow at least one full day (two days is better) for drying before applying a proceeding coat. You will begin to notice a visible increase in gloss after about the third coat. The number of coats is up to you and how long your wife (mine is good for about seven coats) will tolerate having your nose stuck in that piece of wood.
You can continue with tung oil here if you like. Tru-oil and lin-speed (brand names) will give higher sheen and is more commonly used for surface finishing and accessible at about any gun store. As before, the oil will be diluted with mineral spirits and you will need a couple large lint free squares of clean soft cloth. Very little oil/mineral spirits mixture is needed per coat. About 2 tablespoons total mixture should be more than enough to work with. Since the oil that gets into the cloth stiffens when dry, they will eventually need to be replaced. If you can find them, cotton baby diapers are great for this kind of work or sneak out some soft cotton hand towels. Stay with the same oil you choose to surface finish with. If you start with tru-oil or whatever stay with tru-oil or whatever. It is okay that our sealer and filler coats are of tung oil.
Wipe the stock clean of dust. Mix 1 part or a bit more mineral spirits to 1 part oil. You do not want the oil thick. If thick it will get tacky too quickly and not spread evenly. With fingers I apply a small amount to a starting point on the stock. I start at the forend tip on one side of the stock and smear the oil down toward the pistol grip area. When the oil will not not spread any further I wipe it with the cloth in the direction of grain flow . I act as though I am wiping all the oil off but a very thin coat of oil is left behind. Do not be afraid to wipe well and you need to. You want no high spots in the finish and you want all finish to blend together. That is why we want each coat to be very thin. It takes me about four dips worth of the fingers to coat one side from fore-tip to pistol grip. While the oil can be rubbed on in any direction, even circular, it is imperative that the stock be wiped with the grain . Keep working at a comfortable but uninterrupted and steady pace. Do not jump from one part of the stock to another. Rather, keep the work connected. Do not stop until all the stock is coated and wiped down. If spots are missed they can be wetted and wiped dry if not much time has elapsed. After you have finished wipe the stock down again and always in the direction of grain flow. It takes me about a half hour each time I do this, not counting cleanup time. Be sure this time will not be interrupted.
Put no less than four coats on the stock. Keep it out of direct sun light and heat above ambient room temperature between coats and after the final coat until the stock has completely cured (several weeks). On the next to last coat you may want to polish the finish with a super fine polish such as Brownells Five "F" Stock Rubbing Compound. This will smooth the finish. If you have applied quite a few coats, say seven or more, you may lightly polish with Three "F" compound followed by Five "F". Do not polish the final coat as that will dull its gloss. If there are any checkering patterns clean them out with a checkering tool. Brush them clean of dust and apply diluted tung oil to the patterns only. Let dry then apply one diluted coat of finishing oil to the patterns only.
After the final coat has dried the stock is ready for assembly or glass bedding .