Are you referring to the joint at which the flue piping is connected or
a coating on the flue?
The slip fit of most flue piping (different types for different applications) should follow a pattern. Starting from the appliance the fit should be inserted “into” the next segment not “over” the next segment downstream. Following the flow pattern of the exhaust ( combustion gases) ---->>----> Not ----<>----> (these are the ends of the flue piping > into >) .
Just in general , MFG installation instructions are the number one
recommendations to follow.
You can find these in the booklets that “should” be next to any Natural gas burning appliance. If this booklet is not there then the home owner should have it in their possession .
For a great explanation of Natural Gas burning appliances (flue function) see :www.gamanet.org/gama/inforesources.nsf/vAllDocs/Safety?OpenDocument
Goto “Venting Done Right” and watch the video.