Simon
Absolutely serious, raw Tung Oil will not set up unless thinned. For a first few coats, for penetration, I thin to about 25% Tung oil, wash this on until the wood stops absorbing it, wipe it down and let it dry. This is your sealer coat.
Now is when you wet sand to fill the pores with sawdust , while you are washing on the sealer. If you do this use a relatively hard sanding block that will force the sawdust into the pores. You might do this two or three times until the pores are filled. This is necessary with open pored woods such as Walnut and Oak, unnecessary with closed pore woods such as Maple or Birch. Of course, you can use a comercial filler to do the same thing.
Then I gradually work up to about a 60% Tung Oil mix as final coats, some go to 75%, my method is far from precise. Actually, this takes several days, and around here the thinner gasses off the open container at about the right pace. Sometimes when it's humid I have to add some Tung Oil, or when it's dry I have to add some thinner. I do it by feel.
This final mix you can use until you get the finish you want. You can stop at a soft, oil like finish, or you can continue and polish until you get almost a lacquer like finish. This shiny finish may take 10 coats or more, though. Most who want this will use poly, it's a quicker and more water resistant finish.
If you get shiny splotches, this is oil that came back out of pores you didn't get filled, spread around the pore, and hardened. You will have to sand these out and start your final finish again, but do fill those pores. The other solution is to continue to wipe the surface until there is no liquid oil on it, and that can take a while, believe me.
I also try to wear surgical gloves when I do this, it keeps my hands from cracking as the dryers in the thinner work on the skin.