tapping means "peening" the weld and yes you do it right after you do a short section of weld to stress relieve it. Another option that I have actually done with sucess is the small "cobra" torch and use cast iron flux with a cast iron rod. very littlw preheat is needed but you got to have the right flux and rod or it wont work. It also didnot work when I tried this with a regular torch so the cobra torcj is worth it just to weld cast iron. (too bad some one stole my whole welding setup including the mig0