If there is excessive weld metal buildup at the extremities of the tack, then a quick hit of the grinder is recommended.
For a restart, the quicker the rod is changed and restarted, the better. This allows the restart to begin with the weld metal still hot. Normally it is not necessary to hit it with a grinder. When starting, it helps to preheat with a longer arc the weld metal, just before entering the keyhole.
Perhaps others have had different experiences.
edit: There is also the need to consider the acceptance criteria. Some specifications and/or codes have a more stringent criteria. For example certain types of discontinuities, the API1104 use to have a more lax criteria than the ASME B31.3 piping code. If the amount of porosity that remains often at restarts is within the acceptance criteria, then some welders will choose to not hit it with a grinder before restarting to weld.
Last edited by Alexa
on Tue Jun 11, 2013 12:43 am, edited 1 time in total.