On walnut I use the same oil I do for the final finish. Low Gloss Tung Oil finish. Formbys or Gilispies.
I wet sand it with 320 grit to get the pours filled fast. The fist coat will be wet sanded back down to ground before the final finish coats go on, so you need not be super careful about applying it to seal the surface. I am very careful in restoration jobs however, because you don't want to get any in the checkering.
"Sealing" is an old term and it means making the surface flat, in that it will have no open pours acting as small pits. No finish known to man will actually waterproof wood 100%. So "Sealing" is a bit misleading in todays understanding. Like the words "Bluing" and "carding", sealing has a meaning that is not the same in the common vernacular as it is in the gunsmithing world.
There are some finishes that are just a bit better overall then Tung Oil Finish for repelling water, but not much, and none I have found that are as easy to repair.
I have some rifles I made in my teens that have Tung Oil Finish on them and 2 of them have never been refinished. They look fine.
My 375 has been refinished one time. I used to go into the woods with it on very long hunts in late September and some years it would be mostly outside until almost Christmas. That was when i was hunting and guiding in the Selway, in Idaho. Several years of that made me decide to refinish it.
I put on a new finish when I was about 31 years old. It's got the same finish on it now and looks very good still.
Now that finish looks like it is on a gun that has been hunted with for 45 years or so (it has) but all in all, the finish still looks very good.
