Will pressure inside the glass work as well as using a crack expander?

Question:

If I do not have a crack expander, is it okay to apply pressure on the inside of the glass to open a crack, so the resin will flow into the break easier? Will that work as well as using a crack expander?

Answer:

You can use thumb pressure on the inside of the glass to flex open a crack, however, the advantage of using a crack expander is that the crack will stay open until you have filled it with resin and removed the expander. Flexing with your thumb will open and close the crack as the resin is flowing which can cause air bubbles to become trapped in the break.