(R. Hoshiyah): (After the utensil has been kashered) one must still immerse it (in a Mikveh).

As we find when R. Ami went with R. Yehuda Nesiya to the bathhouses of Geder. They asked to borrow tongues of silver from the (gentile) Usinus family.

They asked R. Yirmiyah whether the silver needed immersing, since it would later be returned to its owners - he said that it does, since it has left the domain of the impurity of the gentile and entered the domain of the sanctity of Yisrael. R. Ami and R. Yehuda agreed to go out and learn this topic. They then heard R. Yaakov bar Acha and Shimon bar Ba saying in the name of R. Yochanan - the requirement to immerse the utensils only applies to those that were bought from a gentile but not those that were borrowed.

If so, this even applies to utensils bought from a gentile that had never been used before. This law was observed by R. Hoshiyah.

Question: The Mishnah taught that utensils used for roasting must be heated in fire and in Maseches Zevachim, the Mishnah teaches that such utensils (with which one roasted the meat of sacrifices) must merely be put into boiling water (in order for them to spit out any flavors absorbed from sacrificial meat and render them now permitted to use)?

Answer: They were stringent with the utensils of gentiles (because they absorbed prohibited flavors).

If a knife was used to cut a prohibited food, it may merely be inserted three times into the ground (to scrap off the fat and grease). (Note: According to the Talmud Bavli, ten insertions are required.)

(R. Ba citing Rav Yehuda): This refers to a small knife, but a large knife (which is not smooth) must be kashered, which involves inserting it into fire until sparks fly off.