The kits have instructions about the application of the coating. I
might have a concern if the coating could possibly come loose and wind up
plugging the drain or air outlet.


Considering experiences that I had with paints that claimed to
"chemically bond with rust", I was a little skeptical...

Very true. I've never found a rust-eating paint which works miracles.
The best I've found is "Vactan" rust treatment.

Chris