Poly (varnish) already is tung oil, or at least some approximation of it,
resins, and a carrier (mineral spirits). You can mix in more oil and/or more
thinner and make you own Danish oil or your own wiping varnish, depending on
what you are looking for.
If all you want to do is seal MDF adding tung oil to the varnish isn't going
to do much. Certainly not provide extra protection. Thin the varnish fifteen
or twenty percent to make yourself a wiping varnish and just wipe it on or
don't and brush it on and you'll have all the protection you can get from a
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