Lots of reasons porosity gets in welds, no gas coverage, dirty areas to weld on, oil on the chains or S hooks, poor welding, to much stick out on the wire. Who ever welded them up used about all of them reasons LOL !
I will say this every time it comes up, non-experienced people are always trying to say you need to have gas coverage with a mig welder, this is not true, it really depends on what type of wire your using, most all solid wires require a shielding gas, however there are inner shield wires that do not require a gas coverage, all dual shield wires require it, hence the name dual with the description. Most inner shield wires will run smoother with an added gas coverage though.
Yea those make a seller look real bad if they are just starting out and want to make a go of something
Outside of cleanliness, id bet wire "stick-out" plays a major role in a lot. Then you get into
polarities crossed with wrong innershield wire, etc, etc. So many variables with different wire-
feed applications.
Good post GritGuy.