Since we are concerned that hot Chametz may have been in the strainer, the method of koshering depends on the material.

If the strainer is made of metal (iron or stainless steel), one must perform Irui—pouring boiling water directly onto it from a “First Vessel” (Keli Rishon) while the water is still bubbling.

However, if the strainer is made of plastic, there is a dispute among the later authorities (Acharonim) as to whether this material can be effectively koshered. Therefore, it is proper to avoid trying to kosher a plastic strainer for Pesach and instead use a new one designated for the holiday.