With a Brush and Bucket this is what sealing the interior duct Pitts seam often looks like…not very good.

Feedback is that some contractors apply the mastic to the Longitudinal seam (the Pittsburgh Lock) INSIDE the duct work rather than on OUTSIDE. Reasons we hear are that it’s easier to do Interior duct sealing neatly and control the bead of mastic from caulk tube than it would be reaching inside the duct with a Chip Brush of Mastic dripping on the interior duct wall.