I know of no chemical or additive that will stop phase separation. All fuels with Ethanol will eventually separate. Your only defense is to use non-ethanol fuel, or use ethanol added fuels as soon as possible, before the separation phase begins.

Make sure the fuel lines from your tank, to the motor, and all parts in between are made from a Ethanol resistant material, and replace every 2 to 3 years.

What is the time line here? When you say use it as soon as possible, are we talking weeks?,months?

Some good info here on fuels. Most all racing fuel has some ethanol in it as an oxygenator.
The best bet on fuel storage is to have the tank completely full so there is no air left in the tank. Air contains moisture/water and eventually the moisture will transfer into the ethanol and then later separate. Racing fuels in an unopened drum can be stored for months without going bad.
Pure gas makes more power in most engines except those that are set up for racing. Racing engines that use alcohol fuels have much more timing advance and use a lot richer fuel mixture.

Does the fuel filter act as a water separator or do you need a "job specific " separator for that?

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