Prevent rust from spreading?

[dave73chally]

Bought a 73 Challnger with some rust I'd like to tackle. There is a small rust patch below the rear window frame on the drivers side, trunk floor is rusty but still solid (guessing from the rear window leaking), and bottom of quarters are bubbling paint. Can't afford a new paint job yet, what should I do to prevent the rust from getting any worse for the time being?

[Topcat]

Rust on the rear window channel means water into trunk floor.
which means rust on the trunk floor.

It has to be cut out and a new portion butt welded in. No exceptions. No substitutions.
You've got pitting/holes and that suggests weakened metal.   

Rust Never sleeps: by Neil Young.

[Edison1970]

Cutting out is the only real cure but if you need to put a band aid on it, you need to determine if that rear window actually has a hole under the glass.  I had a 70 challenger that leaked and it was from the old crappy ribbon gasket I used.