Quick tips on stopping the rust on the cheap

My method from 30 yrs of it. Sandblast is obvios best where you do not distort the metal. It will eat away thinner rusted metal. SO you do not have a blaster. Use right angle grinder ( NOT a drill) and large wire CUP, I am looking at flat surface of inner fender as I talk here of your car. You are trying to get into the rusted pits . An 80 grit flap disc on a GRINDER will heat the metal and distort but will be OK on a surface like that inner fender. NOT OK on basic thin sheetmeatl like doors, fenders, hood, etc UNLESS you are careful not to heat up the metal, There you must use a DA or strip the paint with aircraft stripper. . ALMOST impossible with the flap disc at high rev. Since no blaster. TRY this, go to a true value hardware and get a qt of OSPHO, a phosphoric acid solution. $12 plus tax. Soak this area after using cup and wiping or blowing it clean. Keep it wet for 30 minutes!! IN the edges where the wire cup can't get, them use a wire wheel on your drill, Look at this treated area the next day, If you got it decent , it will have some white residue. That is good.
Use this method and stay away from the "rust killing topcoats". Sand, grind, away rust and treat the remaining metal. Then use a good epoxy primer. then, go to your sanding primers, sealers, topcoats etc...
Now you are at the second day after Osho. Clean this area, Wipe it down with something to get rid of excess of phosphate dust. DISCLIMER: If you screw up, don't blame me!!!!!
Ospho is an acid, and understand you can get a reaction with the treated areas maybe and an acid based epoxy primer. This area you have wire brushed , treated with Ospho, it needs a good primer. epoxy or maybe a self etch primer. Quality auto primers are first chlice, but WE have no idea what you have to work with. Any compressor? If not, I would primer with a rattle can from O Reilly's etc, Their best. I have used a Mar Hype etch primer that works decent. $20 a big can. ( Having any compressor and a cheap gun and BUYING a gal of expoxy primer is the way to go though!!!!!! )...... Also some Rust Oleum, brands.....Be careful with ever going over an etch primer with rattle can tractor type enamels like Ryst Olem. Always test a small area to see if you get a bad reaction.
I understand you are experimenting with making your care better, OT trying to do a resto right now. YOU can do a lot of good with making clean metal , treating the rusted minor pits you will find, using a decent epoxy primer, and sealing that with some sort of topcoat. Something like Rust Oleum is very strong, durable, but realize this type paint will have problems with UV rays and keeping a shine.
Good luck!!!!!!