If the iron sticks were used before then it should be like "bishul achar bishul", so what's the problem at all?
israel shnaidman, Jerusalem, Israel
The Nishmas Adam (Shabbos Klal 20,6) proves that as far as metal is concerned, Yesh Bishul Achar Bishul. The Minchas Chinuch (32:11) also inclines to this view.
Furthermore, I am not aware of any Rishon being of the opinion that Tziruf is a Toldah of Bishul, but either of Makeh b'Patish (Rashi as quoted by Magid Mishneh, Shabbos 12:1) or Mechabeh (Rabeinu Chananel, Yoma 34b and Rambam, Shabos 12:2).
Yesh "Makeh b'Patish" achar "Makeh b'Patish"?
Generally, there can be Makeh b'Patish after Makeh b'Patish. See Mishnah Berurah (338:15) that winding a watch is Makeh b'Patish (Tikun Maneh). There are some cases where we only say Makeh b'Patish the first time that the Kli is manufactured (Shabos 48a). This is because here the Chashivus of the Kli is retained even when it requires a further Tikun.