Imo this is not really time/cost effective to do this nor easy. Think about it you're filling maybe 12 small holes per side... 24 different holes to weld, smooth and de warp. Your metal guy would need to be very competent to achieve a flaw free job. There are some other ways to do it...with epoxy base & bondo surface for intstance that are less invasive but still time intensive. I've only seen the welding method done in folly or desperation if the original rear fender is unobtainable. Much easier in most cases to source a correct non skirt guard holed fender.
Here is an example of this having been done, looks like they gave up!!
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