@IanC - If this is how the metal looked when you were welding it, you've got issues. You REALLY need to have it clean. I mean, you scrape it off eliminating all rust/crud/crap. Then wire wheel it, to clean it further. When it looks like it's all bare metal, hit it with some acetone to clean off any oil contaminates. Then you go to welding it. This is on both the car side and the patch side.

@Pᴀᴜʟsᴛᴇʀ2 I concur. While I don't know much about good welding (altough I am an expert in bad welding), I know that it must be spotless clean...

@Martin - Not that it has to be spotlessly clean, but usually the cleaner it is, the better the weld turns out. Less contaminates means better weld. Just like in a paint job, the final product depends on the preparation.

@Pᴀᴜʟsᴛᴇʀ2 You're right, that might be one of the reasons I had a hard time doing the welding (I think I also had some wrong settings at the first ones like wire feed speed). I only had an angle grinder so it was a bit hard to hit those small spots without eating away much of the sheet, but I'm trying to get myself a rotary tool, so hopefully I'll be able to clean it better next time. Can I use paint remover instead of acetone to clean it?

Also, this is the current state of the fix:

I need to finish welding those spots missing on the bare metal part, because it got dark as I was finishing it. Then grind down the excess and weld this black plate plus fill that tiny hole left with some small patch. And finally bondo, paint and move to the other side

Plus on some sad news, after working hard to remove my back windshield without breaking it, it spontaneously exploded. The only explanation I can possibly come up with is that since I was keeping it inside the car, it might have got some heat from the welding process and then the colder night got some thermal shock or something. RIP

@IanC - Sorry to hear about your window. It's safety glass. All it needs is a bit of stress on it and it'll pop. I doubt it was anything other than it not being in its frame which caused it. You could have had one little scratch on it from somewhere and a little bit of a giggle and boom ... it's all over but the crying.

And if you don't have acetone, something like brake or carb cleaner will work just fine. I wouldn't use paint remover as it isn't anything like what you need. I think it would leave some crud on there.