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 but the advantage of using a crack expander is the crack will stay open until you have filled the crack with resin and removed the expander.  Flexing with your thumb will open and close the crack as the resin is flowing into the crack which can cause air bubbles to become trapped in the crack.