The two things that you need to look for are
- Glazer points
- small bumps of putty stuck to the wood
Cut down. Take your time make several passes don't rush don't push against the glass. Make sure their are no bumps of putty that will hold in the glass. There should be a knife thick grove between the window and the wood. Watch out for points dulling the razor.
When i have broken the pains it was because of these bumps or hidden points.