I've got a new woodworking project planned out.  This time around I want to try a Birchwood Casey Tru Oil finish.

I've heard a lot of good things about the stuff and seen some impressively finished gun stocks that were done with it.   I think it'll deliver the deep and full gloss finish I want to achieve.

I get the basic idea.  Like any other finish that's applied in thin coats,  you build a couple of coats, sand them flat, build more coats, sand flat again, repeat until all imperfections (wood grain included) are filled in and the surface is glass smooth without pits, voids, or indentations of any sort.   Not exactly the fastest possible finish process, but if done right it'll look great.

But what do you actually use for your applicators and also what do you use to polish out the finish when you judge that it's ready for that?

Oh, sure, I've heard about the "classic" old school way of using rottenstone and mineral oil,  but what about more modern polishes?  Do any of them work on tru oil?

True oil is pretty thick, If you're application is furniture or other non firearm type of deal, I would thin it out before application, and wet sand progressively finer grits) with oil in between coats.

Would NOT recommend on any rifle other than a Fudd gun.

If you really want a stunning finish, google up 'French Polish'

I've done some stocks with it.  They came out nice.
For applicators i just use a lint free rag. I usually put one buttplate screw in and hang it on that some place out of the way.  It is a long process.
Make sure you "whisker" the wood good before hand. Then stain.
After 2 or 3 coats i use 0000 steel wool.  Clean that off real good then repeat until you get the finish you want.

I'd say practice on a cheap gun first.