mig welds especially need to be clean,the pin holes you see are porosity.You will have to grind the crab out and refill

-oil,dirt,paint,rust and improper amount or wrong sheilding gas cause porosity

-on bigger gaps position weld area vertically and weave top to bottom until edges are joined then lay flat to completely fill

- a pre-heat will help with distortion of piece and burn off crap

-your mig should have a tag on the inside of its door describing voltage,wire speed and proper shielding gas for type/thickness of metal

-although mig is the easiest method of welding it is not the most ductile ,stick welding is better for strength, 7018 rod is best,DO NOT USE FLUX CORE...brittle and ugly weld