Why should a round bead not be used in urethane application?

Using a round bead in urethane application is discouraged primarily because it can trap air in the corners of the joint or seam being filled. Air entrapment can lead to several issues, such as poor adhesion and compromised structural integrity of the seal. Proper sealing requires that the urethane material contact the surface evenly and without gaps. A round bead does not facilitate this, as it may create uneven surfaces that leave pockets of air.

This is critical in applications where a strong, watertight, or airtight seal is essential. Therefore, when applying urethane, it's important to use a bead shape that promotes optimal contact and minimizes the risk of air being trapped in corners, ensuring a successful and durable seal.