I have done this several times and it works fine. Use only denatured alcohol. Rather than mixing equal parts of epoxy and alcohol, use only enough alcohol to thin the epoxy to a liquid state. It takes very little alcohol.

For just a firewall, you probably don't even need to bother with the alcohol, just use 30 minute epoxy and apply with an old toothbrush. Scrape the excess, if any, off with an old plastic card and if it's not smooth enough for you just hit the wet epoxy with a hair dryer, that makes it lay out and gloss up real nice.

While many seem to get passable results thinning with alcohol, it is not a good idea. If there is to much water in the alcohol, the result may be a soft or rubbery finish with little strength and; all alcohol will have water in it, even denatured alcohol will absorb quite a bit of water from the air after it has been opened a time or two. The advice to use 30 minute or slower epoxy and then use a heat gun to warm it is a far better solution. The warmed epoxy will seep into any crack or crevices as well and greatly strengthen the firewall area. Just don't heat it so hot that that it bubbles, just enough to make it very thin (you will see it suddenly being absorbed into all the little cracks and crevices). The heat will also speed up the cure and prevent any globs or ridges of epoxy.

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.

I've used thin CyA on all of mine, and it works even better. It adds strength to the firewall, and no fuel can get through. Another way to do it is to get some paint and paint it, then spray ClearKote over it.