Doesn't matter. When the gas evaporates it leaves two stroke oil in the engine, not "varnish". Unless you're using a castor based oil, in which case you'll have more varnish to worry about just from running the engine.

The oil is both a lubricant and a protectant. You can run an engine and put it away for several years with only placing a bag around the engine to seal in the residual oil. When it comes out of the bag the engine will be just fine a few years later. I've got some engines here (more than 20) that have been sitting in a box for years after their last runs. All of them still have a adequate oil film in them after their last runs. Don't even have bags on those...