Hand rubbed modified tung oil finish. No surface finish applied.
Filling the grain will smooth the stock to a slippery surface. It enhances appearance and helps to weather proof. This is the step that will allow the finishing oil(s) to bring out the full beauty of the grain and its figure.
Before filling the grain the stock should be sealed . You will not be able to get a sealer deep into the wood once the wet sanding process begins.
The stock will be sanded with fine grit wet-or-dry paper wetted with oil. Modified tung oil is a classic choice for gun stocks and I will proceed as if that is the oil being used, although the procedure should apply to other popular gunstock oils prescribed for filling grain. It is pretty simple to do but requires a soft touch. The object is to sand wood dust into the oil. When the oil becomes thick with wood dust sanding stops and the oily sludge is gently wiped across the grain causing the grain to be filled with its own oily dust.
We will thin the oil so it will not too much impede the sanding effect of the paper. Mix 2 parts mineral spirits with 1 part oil for use with the finest grit paper that will add sanding dust to the oil before the oil gets too tacky. It will not take much per session. About 3 tablespoons total mixture should do it but start out with more at first until you get the feel for it. It won't hurt to mix a little more mineral spirits either. Cut the paper into small squares or rectangles. One inch by two inch or two by two should do it. Have several of them ready, say about 10 or 12. Start with 400 grit wet-or-dry paper moving up to 320 grit if needed. If starting with wet-or-dry 320 grit paper go down to 400 grit after the wood is mostly filled. Be sure to have about two squares of lint free cloth on hand to wipe the sludge into the grain. I sometimes use toilet paper if nothing preferable is handy.
Wet the paper with the oil/mineral spirits mixture and sand with the grain (in the direction of grain flow) and always us a sanding block wherever possible. Be aware of how the grain flows at all parts of the stock so you never sand against it . Sand a small area at a time until the oil becomes thick with dust. With a little experience you will feel and see when the mixture is prime for wiping into the grain. Gently wipe the sludge across the grain diagonal to its direction of flow (close to opposite the direction you were sanding). To do this, I use not much more than the weight of the cloth. Done with the right touch, this will force the dust thickened oil into the grain. Do not try to wipe the oil off the stock. That will pull oil out of the grain. Continue wet sanding and filling relatively small adjacent areas until all the stock has been done. The stock will be thinly smeared with the dull oil. 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. Depending on the wood, it will take several wet sanding sessions to complete the filling process and it is essential that you allow the oil to dry before wet sanding again. Otherwise uncured oil will be lifted out of the grain. Some walnut may take 6 or more attempts to get all grain level. Sanding too forcefully can open new grain making it more difficult to fill the stock.
After each wet sanding and the stock is fairly dry inspect it under various light conditions to see how well the grain is filling. Check it from different angles in both dim and bright light. You will notice that the grain appears more filled before the oil dries and that the oil seems to collapse onto itself as it dries. This will lesson with each successive wet sanding.
Some grain, especially on highly figured wood, may not fill completely. If some grain just won't completely close you can try something else. Use a sharp pointed tooth pick, or something else that is not harder than the wood, tipped with a very small amount of undiluted tung oil to fill tiny openings in the grain. Let dry then polish smooth with a rubbing compound or 600 grit to 1200 grit paper. If a cluster of grain won't fill you can wet sand it separately but sometimes this opens some grain at the outskirts of were you sand. Another way that has worked for me is to wet sand another piece of the same type wood (walnut, whatever); wipe up the sludge and transfer it to the area to be filled.
Keep the stock 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).
When the stock is filled and completely dry (no less than two full days to dry and longer is better) it should be polished smooth to the surface of the wood using a medium to fine grade polish such as Brownells Original or Triple "F" Stock Rubbing Compound. After the stock has been polished smooth and to the wood inspect it for defects. If no defects are found rub the stock down well with a finer polish such as Five "F" Stock Rubbing Compound to enhance sheen. Clean out any checkering patterns with a checkering tool. Brush them clean of dust and lightly coat the patterns only with diluted tung oil. At this point you have a 'classic' hand rubbed tung oil finish with all the oil inside the wood and none on top of the wood.