Why should metal be wiped clean before applying mastic?

Field answer

You apply a layer of bucket mastic over a newly pre-assembled TDC companion flange joint, but skip wiping down the metal skin, causing the water-based sealer to roll up and slide right off the thick layer of factory stamping oil.

Sealant will not bond to oil, water, or dust. Clean and dry the joint first, then reapply mastic to a mechanically closed seam. The likely recovery is to check the condition, correct prep/setup if it is within your assignment, and bring the foreman clean information before the work creates rework overhead.

What to check first

• Look for oily sheen, dust, or condensation on the metal.
• Wipe with approved cleaner/rag per site practice.
• Let the joint dry before sealing.
• Reapply a clean, even coat.
• Check adhesion before covering or testing.

Why it matters

Bad surface prep creates leaks even when the joint looks sealed at first.

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Final direction belongs to the foreman, approved drawings/specs, manufacturer instructions, pressure/material schedule, employer policy, and AHJ/code requirements.