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.