If it was me, I'd just use a plastic tube and a five gallon can to syphon it dry. Pour the varnish/gas out in to a field somewhere. I'd just make a couple of trips, and I would not put a lot of effort in to this -- if you get the gas level down to where a plastic tube doesn't syphon any more gas out of the tank then you'll be fine with a full tank of fresh gas.
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