I try to maintain a sharp edge at the top of the chamber or in the case of vintage Comoys the inner beveled edge of the bowl. What I do after every smoke, before cleaning with a pipe cleaner, I'll clean out the chamber with a twisted paper towel. Or a smaller bit of paper towel inserted into the cleft of an old fashion wooden clothes pin. Then I'll take this to the inner edge till its smoothed down to the wood. In this way my cake ends abruptly about 1/8th to 1/16th below the rim. Just looks,neater this way. Of course on old scorched rim estates, I'll top the bowl till I have nice clean edges to work with.