Epoxy will age and deteriorate and after a certain period there is no good or bad mix. There just isn't any mix that will work. Get some new stuff. Remove the old from your firewall with denatured alcohol as much as you can. The removal is very important because the old uncured junk will still be old uncured junk under the new
material. Rodney is certainly correct about the difference between 5 min. & 30 min. epoxy. You don't have to wait to get more epoxy by mail order. ACE hardware as well as any other complete hardware store probably has something you can use. Just check the cure time on the package before you take it home.