rust fixable?

How do you go about fixing that?

Cut it out and weld in a replacement.

Either find a rust free donor to cut clean pieces from, or make your own.

And fix the probable water leaks in the T-tops that caused those areas to rust out.

For an example, this guy is doing a shit-ton of rust repair on his Z31.

http://z31performance.com/showthread…gian-Z-project

Do I use sheet metal or another type? Never done body Work so I'd obviously have help but trying to learn. Thanks.

The only thing you can do is cut it out and welding new selfmade/donor parts

You take sheet metal. 1mm is enough. But I uses 2mm in the bottom.
You cut it out and you made a new part with the same shape.
In my topic you can see the same repair that you need to do.