Agree varnish is not corrosive. I let a sample of gas from my tank evaporate and what was left was a golden sticky residue - likely varnish. So my best guess is that if that fuel is run through the engine and then the engine is left to sit (36 hours was the record minimum) that the fuel pump impeller and impeller housing would get coated with varnish after which it would exceed the fuse rating (5 Amps) in current when trying to start up.
In terms of the quantity of varnish, not knowing what the history was of the fuel run in the boat, and in general not being aware of the dark side of ethanol, I was adding E10 fuel to my tank in small quantities, hence a potentially high ratio of varnish to fuel. Its possible that if I had filled the tank completely that the varnish would have been sufficiently diluted to be rendered harmless.
In terms of the quantity of varnish, not knowing what the history was of the fuel run in the boat, and in general not being aware of the dark side of ethanol, I was adding E10 fuel to my tank in small quantities, hence a potentially high ratio of varnish to fuel. Its possible that if I had filled the tank completely that the varnish would have been sufficiently diluted to be rendered harmless.