I believe some tiny pinholes are inevitable. I think (not sure) closing pinholes with more weld may just leave other pin holes (I don't weld, so a little ignorant here, just my observations watching others).
Well that would have been a real problem when I welded up a big hole in a trans pan.
Cleanliness, temp, wire type and shielding gas is the solution.
When I weld something like that I put gas or lacquer thinner in it afterwards to check for leaks, since they both will show any leaks right away.
