Add 8 oz. cultured buttermilk on the top then cover with a large cabbage leaf or two.
I've never heard about using buttermilk.
I have been making sauerkraut for many years....I gave my friend a 6 gallon brew bucket with airlock, and he now makes it to...no problems....as stated just weigh it down somehow and cover with 1-2 inches of liquid above the cabbage and you will be fine. I Usually never have enough liquid so I will make up some Brine, with boiled and cooled H2O (15g Salt for 1L H2O). Make sure that no pieces of cabbage are above liquid, or stuck to side of vessel....I usually wipe insides down with clean paper towel so no bits above liquid. I also use several big cabbage leaves between weight and chopped cabbage (still under brine or liquid)
I Got a German Made Harsch 30L crock and the book says 5- 8 grams (15g max) of salt (non-iodized or Kosher) for every 1kg of cabbage. From what I've gathered... the the warmer the ferment temp the more salt you use to prevent spoilage. I typically make sauerkraut in cooler temps so I tend to use less, and prefer the taste of a less salty product.
The Harsch Booklet also says to start out fermentation in at room temp (68-71F) for 2-3 days (until fermentation starts), and then move to a cooler (59-65F) area for 4-6 weeks. After that store it between 41-59F. I usually test it at about 3-4 weeks because I like my kraut with a little more crisp in the bite texture...and I like the taste of it when its young....
I make about 5 gallons at a time to last through-out the year.
By by the time i get to the last of it, the kraut has softened and changed flavor since storing....but still good.
I have also frozen some to see how it would turn out when thawed after couple months...I didn't find any noticeable difference in the product....BYMMV.
Remember Fresh Sauerkraut (not canned or cooked) sauerkraut has all those good GUT BUGS in it that yogurt does....and those BUGS are supposed to be good for you ....even prevent the Bird Flu (Avian Flu) ...google it!
but again YMMV....