Over a period of a few months I've ended up with 6 MP brass molds, and like others have experienced over the years, got tinning on the mold faces. Part of the problem was obviously me as the molds cast so good I kept going and going to find the gunk in a few places after a long session. I wouldn't notice it until I looked at under a lighted magnifying glass. I've seen it on all of my brass molds to one degree or another. Every mold but one had been heat cycled multiple times before first use. One was thrown into the fight new to see if it makes any difference for me to go to the trouble of heat cycling. Hint: it does for me but the tinning is still a problem.
As I'm casting I will keep an eye on the mold faces and wipe them either with my glove, or heavy duty bamboo Qtip (gun cleaning size) and this helps. I've even bought the Liquid Wrench dry lube and tried it (it helped somewhat and now use it on all my molds). However there is the tinning again after I look under the magnifying glass. I went through the tin seed removal process desribed many times on this site and can get it off, but wanted a better way if there is a such of a thing.
I've also read where some have never had tinning with these molds for any number of reasons. I have and do have the issue, though it has lessened somewhat after many casting sessions. Someone mentioned liver of sulfur, someone else mentioned pencil lead, others mentioned whatever to coat the mold faces. I recently tried coating the faces with Brownells 44/40 cold bluing and gave it a go. So far it appears to work perfectly. Now with the mold faces being a light brown color, if I were to see any build up begin, I'd know it and wipe it off while the mold is hot. I don't find the tin removal to be a fun expenditure of time.
These molds cast great looking bullets that just pile up quickly and are so easy to use. That said, I have many aluminum and iron molds that I just don't have the tinning issue with. Same alloys, same pot, same everything except the metal the mold is made from and temperature/cadence of casting due to the mold alloy.
Has anybody else resorted to coating their brass molds recently?