As long as its not a water based varnish and the temps are at least in the 40's it should be all right, but I agree with RT, why take the risk?
As far as warming in the microwave goes (If there's no sparks and it doesn't catch fire), that might help the varnish be at the right consistency in the pot, but once applied in the thin layer on the wood, it will cool to the ambient temps in seconds.
Ken
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