Now say i cut out the small section.. like say 2" x 2". Weld in the nice peice of sheet metal. Now whats next?

What I did to one of the holes i had when i was taking the spot welds out of the back seat bracket. I went alittle to far down and yes went through. So cut a tiny tiny peice of sheet metal. put it in the center, then mig welded around . then grinded smooth. after that i just couldnt get the microscopic pin holes to go away. So i took some body filler. Applied over, sanded. then primered with 2 thick coats. Will that last?

Also is there anything you should do to the underside of the car when you weld a new peice of metal in.