Just a couple more thoughts. MDF would be the worst possible choice for panel material in my opinion, because being compounded of wood flour, glue binders of indeterminate composition, and floor-sweepings, it is neither structural nor does it resist the absorption of ambient humidity even as well as solid wood.
Vacuum packing would be a solution if the painting were never to be exposed to a climate other than the one in which it was painted. The slower the change to different ambient conditions, the better - hence, a "slow boat to China" would be theoretically better than air freight!
Curiously, ambient humiditiy does not affect the "drying" of oil paints and the varnishes and solvents normally used with it. Oxidation is the process which "dries" paint, and while high humidity may affect the rate which solvents and volatiles "off-gas" in the process, oftimes, paint will actually "cure" better in the presence of some moisture than in a bone-dry climate such as in the Arizona desert.
Finally, one of the properties of paints which has enabled such coatings to preserve wood and other materials, is the fact that the vehicles employed are innately pretty good fungicides/bactericides. A relatively fresh painting will be unlikely to "grow" stuff . . . terpenes and other volatiles would prevent that. To this end, it would be advisable to apply a coating of an oil ground to the back of any painting support. The open backside of a canvas sized with hide glue would present an ideal medium for growing "things", however!
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