Do not smear mastic over the outside of a metal-to-metal flange leak if the missing gasket is inside the joint.
Check
Listen for whistle at the sleeve perimeter.
Check if gasket tape or sealant is actually between the faces.
Look for warped flange corners or uneven compression.
Clean oil/dust before resealing.
Retighten evenly and retest before cover.
Steps
Listen for whistle at the sleeve perimeter.
Check if gasket tape or sealant is actually between the faces.
Look for warped flange corners or uneven compression.
Clean oil/dust before resealing.
Retighten evenly and retest before cover.
Say this to your foreman
The duct-to-louver sleeve at [location] is whistling and appears bolted metal-to-metal. Do you want me to open the joint and add the approved gasket/sealant detail?