Rust / Rot : How to deal with it

I will agree about using the Harbor Freight welder once and never using it again.....lol
The thing about China tools is that R&D means Receive and Duplicate instead of Research and Development.
Like the line from the movie 'Tommy Boy'.... "Because they know all they sold ya was a guaranteed piece of crap. That's all it is, isn't it? Hey, if you want me to take a dump in a box and mark it guaranteed, I will. I got spare time. But for now, for your customer's sake, for your daughter's sake, ya might wanna think about buying a quality product from me.".....end quote

You can use the modern glues such as Fusor, etc. and over lap the panel. But try not to overlap and weld in a panel. Moisture forming between the panel over lap will cause rust to start from the inside out. When you glue in panels the compound will fill in the "air" void and the panel should not rust.
However, the best way is to butt weld the panel in. This way you will not have an over lap to deal with, which means less body filler. I'll attach a couple of pictures from a 32 Chevy we built last year. This will show how the panel should fit and how it can look after the repair is finished in bare metal.