Do not brush more mastic over wet or oily metal and expect it to bond.
Check
Check for factory oil, water, dust, cold condensation, or wet insulation.
Remove loose/failed sealant instead of sealing over it.
Use approved cleaning method/material allowed on that job.
Let the joint dry before resealing.
Verify whether insulation/vapor barrier is needed to stop the surface from getting wet again.
Steps
Check for factory oil, water, dust, cold condensation, or wet insulation.
Remove loose/failed sealant instead of sealing over it.
Use approved cleaning method/material allowed on that job.
Let the joint dry before resealing.
Verify whether insulation/vapor barrier is needed to stop the surface from getting wet again.
Say this to your foreman
The mastic at [location] is peeling because the metal is oily/wet. I checked surface prep and condensation. Do you want the joint cleaned/dried and resealed with the approved sealant, and should insulation/vapor barrier be corrected too?