Ive used cigar ash/water pipe mud in several briars and cobs over the years with zero issues. I did use the plaster of paris for the outside of a couple "raw" MM cobs years ago.....I do not like the bare cob.
About 18 years ago I used the cigar ash mud on a HUGE burn out in a Dr Grabow I got in a lot buy from ebay.
The pipe has been smoked and reamed many, many, many times over the last almost two decades. After a reaming, the repair is 100% unnoticeable.
I have never even heard of the other recipes mentioned here. Now im wondering if there is a difference. I mean, 100% unnoticeable after 18 years of moderate use is pretty hard to beat.
As far as bowl coatings, I have spent time removing the dang things from the few pipes ive bought that had them.
I don't think it matters either way, but ive "broken in" a hundred plus bare chambered pipes over the decades with zero issues.
Again, I do not believe there is a big downside to bowl coatings but when I was a "newbie" in the early days of my internet pipe journey, it was all the rage to hate on and remove bowl coatings. I just followed the advice of the elder pipers. I believe I would just leave the coating now.