Very hard to sufficiently clean the CI from the ingrained carbon after years of use. And the CI often cracks afterwards adjacent to the weld.
Usually when applying sealant to a flanged area it is best to just spread a small bead along the center of the surface and around the bolt holes. Have you ever pulled chunks of silicone caulk off of a partially blocked oil pickup screen? Where do you think that came from?* Less is more!
*(Hint) Overzealous oilpan gasket or valvecover gasket replacement maybe?
Gasket
Here's one I didn't reuse, its a multilayer steel (at least two layers). Looking at the surface of it, it may have had an additional screenlike layer, not sure.
P1040279.JPG
P1040280.JPG
EDIT, LATER...
Now I'm thinking you should just do something like this:
How To Make Custom Exhaust Gaskets For Your Special Project