Good morning Dear Rabbi,
Is it permissible to sieve hot milk to remove the skin which forms on it. Obviously, the milk was in a thermos container before Erev Shabbat.
If it is permissible or not will depend if removing the skin on the top of the milk is something that people consider a preference, and although they would rather the milk be skin free, if it has the skin, they will still drink it. Or, if people consider the milk undrinkable, and won’t drink the milk that way. If it is only a preference then it will not be borer and you may use the sieve, however if it bothers people if it is there then it would be considered borer to use a sieve. (Note: Even if most people wouldn’t mind drinking the milk with the skin, id you are sensitive to it an won’t drink it this way that it is not permitted for you to so it).
Even if the latter is the case, and it would be borer to strain the milk of its skin, you may still remove the skin from the milk with a spoon, by scooping up the skin, (with a spoon and not a sieve) together with some milk. This way you are taking good and bad together and it is permitted.
O:CH 319-10, Taz ibid 3, M:B ibid 61,62, Aruch Hashulchan ibid 33.