ah , so basically , if you cant weld properly then dont hit your joints with a hammer ....you will know this already....but strength reduction is when there isnt full penetration. If butt welding a sectioned repair with mig, often even running a continuous bead is a problem in itself, so what chance is there of a completely penetrated 100 per cent seam....in practice it might be 'good enough' (and often is - especially when there's no easy access to the rear of the panel anyway). if there is incomplete penetration then grinding the bead makes the seam weaker, BUT....
if there's no access then hammer welding in Tig isn't gonna work anyway (unless for spot welding but that's not for sectioning) we do what we can do and the reality is that sections can't always be done by the book...but the OP was talking about 'finesse' (which could actually mean a million different things anyway) but that was the thing I picked up on
