A full weld is made over the hole, and due to the presence of the backstop, burn through becomes less of a problem.

The piece of aluminum that comprised the backstop has been pitted lightly, so use a piece of scrap.

This image shows the backside of the weld and the flat surface that is created. Nothing is perfect, but this trick is worth using for the dramatic decrease in finishing work. The aluminum also helps to draw heat from the steel and decrease warping.

If you require a perfect finish on the backside, I suggest a quick weld to flush the backside and fill any micro-pits. Doing this with the heat of the first weld will allow for a shorter weld time, and resultingly, less finish work.