I have a 2nd gen Rx-7, that I bought and was sitting for a while and has some rust. Also, it the frame where you put your jack on isn't too badly rusted but I am not able to use that to raise the car because the metal is too flismy and it dents it when I apply any kind of pressure with a jack. But the main rust areas where there are some eat away holes on the edge of the underbody.

I have been looking for a while on rust maintanence, and how to clean it up and prevent it. But I have been unable to find anything on how to deal with holes due to rust on the underbody.

Does anyone know a good way to strengthen the frame metal and patch the holes?

 

You're gonna need to be a little more specific about where the metal is sagging.
is it actually the frame rail that's sagging or is it the floor pan?

As for dealing with rust. you need to either buy something like POR-15 or get a stripper disc and an angle grinder, then attack the rust that way.

But from the way you describe it, you will probably need to do some welding, and let me tell you structural frame welding is something that NEEDS to be done right the first time.

 

I believe you can still buy all the sheet metal from mazda, so depending how bad the rust is you can either patch it or replace whole sections.