On a quarter with long a long seam using a stitch welding practice, there's always some porosity/pin hole. Unless you repetitively keep going over those areas. That's where leading comes in. It will flow into those areas, whereas filler may just skim over them.

You want a solder with a large temperature differential wrt Solid & Liquid states.
Soft enough to paddle move, without flowing off.
But with a reasonable tensile/shear strength.
That's usually means middle ranges: usually 30/70 > 50/50.

WRT surface prep: A real ruff surface, while a plus for filler, not needed for lead.
Media blasting, while useful; requires very good cleaning prior to tinning.
Otherwise there will be voids in the tinned finish (i.e. embedded particles).
Just visualize a typical copper pipe joint prep: clean but not ruff nor mirror polished
WRT weld seams: Only MIG, No Flux-Core (again: cleaning of voids).
Wiping: Any good solvent which doesn't leave residue.