I avoid mixing alcohol wth epoxy for any purpose. If you are careful about applcation, and have fresh 2 hour epoxy, it goes on thin enough already for fierwall fuelproofing.

While you are coating the firewall, If the plane doesn't have cheek cowls, and IF you can peel back the covering a bit, put a layer of .5 oz fibregalss cloth over the firewall and back on the fuselage sides, top and bottom for about 1/4 to 1/2 inch. This will ensure the fuelproofing coat is adequate (by all the glass cloth being saturated and changing from white to transparent) and lock the firewall to the fuselage. The fibreglass will be strong enough to hold the firewall in place even if the ARF manufacturer didn't get enough glue on it.