The general Law:
One who smears a poultice, which means that he smoothes and levels the grooves that are in it, as well as one who smears wax or tar and anything of the like which is a material that can be smeared down until it smoothens its surface, then this is an offshoot of [the] “smoothing of hide“ [prohibition], which is [a prohibition against] scraping the wool or the hair off the hide until its surface is smooth, which is a primary prohibited action being that this was done in the Tabernacle as explained in chapter 302 [Halacha 15].
Plugging a hole with wax: Therefore one may not seal a hole with wax or similar things even if one does not enter the wax into the hole but rather only places it onto the opening of the hole in which case it does not appear like one is fixing a vessel with this stuffing being that the stuffing is not entering into the thickness of the walls of the vessel of the hole area and rather is only on the opening of the vessel on its back. Furthermore even if this wax is not prohibited to be moved such as in a case that it was prepared to be sued from before Shabbos, nevertheless it is forbidden to seal with it due to a decree that one may come to smear the wax to attach it to the walls of the vessel which surround the hole to the point that he smoothens its surface there and will then be liable for “smoothening”.
Plugging a hole with fat or congealed oil: Even with fat or congealed oil it is forbidden to seal with due to a decree [that one may come to seal with] wax.
It is forbidden to connive here to [plug with the fat and congealed oil and tell others that] he only intends to hide [the fat in the hole] even if he is a Torah Scholar, as since with wax it is very easily possible for one to come to be liable for a sin offering, [meaning that it is very possible] that he will forget and will smear it in the way it is done usually during the week, in order so that the sealing be well firm, therefore [the Sages] also prohibited [sealing] with fat and oil due to a decree [that one may come to smear with] wax and the like.
General Summary-The Smearing prohibition:
It is a Biblical prohibition to smear and consequently smoothen wax and material of the like. It is thus forbidden to stuff a hole with wax or even congealed oil and fat due to a Rabbinical decree.
Food: Food does not contain a smoothening prohibition, unless it is impossible to eat without doing so in which case it is Rabbinically forbidden. However one who is stringent by foods which are commonly smoothened is blessed. This does not apply to spreading spreads on bread.