You almost can't put it on too thin. I use a 2"x2" square of T-shirt with a cotton ball tied up inside; saturate it with Tru Oil, gently squeeze out the excess, then cover with a thin layer. If you hang the guitar while finishing, you should apply it thin enough that you get no runs whatsoever...not even close. If you have even the slightest run, you're putting it on too thick.
It cures by oxidation, which means the air needs to get to it. If it goes on too thick, or even too many layers too quickly, a skin will build up that prevents air from getting in, and you'll end up with gum.
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Do you have a recommended finishing schedule?
Bill