When is it appropriate to apply urethane to the glass?

Applying urethane to the glass is most appropriately done when the old urethane is present around the edge, the frit is marked, and/or there is a guide channel. This indicates that the area around the glass has been prepared and is ready for new urethane to create a proper bond and seal.

The presence of old urethane may suggest potential problems with adhesion if not adequately managed, and marking on the frit can help ensure that the application is aligned correctly, which is critical for the glass to function properly and maintain its integrity. In instances where a guide channel is present, it can aid in properly placing the urethane, ensuring that the glass is secured effectively.

Understanding the conditions necessary for applying urethane in these specific contexts ensures an optimal bond and effective sealing of the glass, which is crucial for safety and performance. Other scenarios mentioned may not provide the necessary conditions for successful urethane application, as they lack the considerations for proper bonding, alignment, or preparation of the area.