Pure tung oil will eventually form a film, but it will look and feel more like a gummy residue than a clear coat. When its not applied in excess, the finish is just what has penetrated the surface of the wood. I find it wears better than films and tends to be more water resistant, though that is so relative that its hardly worth arguing about
- the reason I say that is because it's softer and more flexible, and not a film, so it won't crack as the wood swells. Wood will pick up water vapor and swell regardless of the finish you use. In the extreme, films tend to crack and flake - though gunstocks rarely seem to see that kind of extreme.
The main advantage is that if you want a nice satin finish, it's about the easiest to apply and maintain. You can put a glossier film oil over the tung if you want, or you can wax it, or just leave it as it is. Wax will make it shiny, but not as much as Tru-oil, and will also need to be renewed now and then.
You don't need to add anything to the tung oil. I'm not familiar with the brand you're using, the two I use regularly are fairly thick and don't smell like an added solvent (like paint thinner). It'd probably be a good idea to do some test pieces and see how your finish behaves. Some "tung oil finish" products have lots of driers added that make them behave more like Tru-oil, and a lot less like what I'm describing. There's no real advantage one way or the other IMHO, I use both types on a regular basis, but they both require different techniques.
Clear as mud? Like I said, some test boards would be a good idea, hardwood rather than a 2x4 if you have some. Or you could put a couple coats on the stock and see what you get. If it starts acting like polyurethane around the third coat, then you'd want to buff and tack between coats, but if it acts like a cutting board and veg. oil then just keep working in each coat with fingers or a rag and wiping off any excess.
