I use stuff called Zap. It is not called Zap-a-gap. I don't know if it is different than what you are all talking about. It comes out of a black tube as a gel and is the best stuff I have ever used. It stays right where you put it instead of soaking throughout the fly and making it hard. It also dries quickly. I have had to run a fine sowing needle down the applicator once or twice but not a big deal. Highly recommend it.