It doesn't look anywhere near that much in the photo. There should be no rust left at all. Just shiny, clean, sound metal to weld to.It might look bad, the edges, but actually at least 80% of it is back to good thick metal. Not all, but that much.
No, but thin, rusty steel melts more easily than full-thickness, non-rusty steel.Could it be that old steel melts easier than modern steel at all?
The gap is a big part of the problem. Do I understand correctly that you have "stepped" the edge so that the edge butts against the underside of the original panel and the centre of the patch sits below? The idea of the step is so that the step sits under the original panel and the centre of the patch sits up flush with the panel. i.e. it looks like your patches are inside-out. Or have I got that wrong?The main problem is the gap, I think, as pointed out. I've made a 'step' with that flange and the old steel sits on the step. The lap joint is out of sight down there on the step. At the top I've got the shoulder of the step and the edge of the old steel. That's why the 'gap looks huge..' as you say, schnappi. Because it often is huge. Between the old edge and the new steel shoulder. The theory was that I've got more metal everywhere, more heatsink, easier welding.
Thanks for all of that, Keith. How to do a 'plug weld' of these screw holes? I have a problem filling small holes - such as blow throughs - that'd be much the same as screw hole.