SLOWING down the rust monster..........

Like said, there is only one real cure, you know what it is
This sheet metal rusts from both sides. Many time just from the inside, like at the rear lower quarter. If you start digging at that rust bubble, you will probably get a hole, and driving the car, if you leave that hole moisture gets to it, and more rust. Leave that bubble and it continues to rust.
Anything you do beside do it right will buy some time. When trying to seal this metal, USE the rustoleum in a qt can and a bursh. Put on several good heavy coats and let each cure, it can be slow. Key is having a heavy coat to to seal it. Problem is rust will be under that paint. Some people use the rust convertors, I clean the metal best possible, and spray Ospho ( a phosphoric acid solution that works for me), Costs bout $18 a qt.
On that truck extension ( that lower rear qtr) yes there is a drain and a plug with a hole in it. Pull that plug and hose it out., as is for the cowl, and drains in the bottom of the doors. Hose them out good too. .
I have an old car buddy (Pontiac guy), he keeps ATF on the floor of his old firebirds. Works for him. Messy nasty no doubt, but he owns an old car boneyard. You understand his logic.