Here is my technique for what it is worth. Many times you can't open a crack without making it worse or only minimally, so getting epoxy in very deep is tough. I use original Acra glass or any quality "thin" long setting epoxy. As already mentioned long set is key and much stronger. I use a shop vac and tape the nozzle to the off side of the crack. They usually go all the way through. Seal it as best you can. Mix your epoxy at at least 70 degrees so its thin, thinner the better. Start the vacuum and start feeding the epoxy in on the off side. You will know right away if its going to work as it sucks it right in. I use a toothpick for small cracks or a small syringe for larger. The vacuum will disperse it through the crack. When it is all the way through and dispersed, remove the tape and hose. Clamp it. Never had one fail yet.
