I've done all kinds of body work but am not a "body man" as in its not my business. I am a rust phobic anfd bought a pressure sand blaster 6 years ago and have done may undercarraiges, which is where I alway start because so does rust. With any sandblasting you need to protect your windows it will pelt them up the stuff bounces in every direction, dont take chances I did ONCE .
before the blaster I used a wire cup on my grinder it does the best to get into pits, sanding or grinding wont untill the surounding good metal is nearly gone. Wire cups on highspeed grinders are dangerous and one should wear goggles and enjoy pulling the wires out of ones shirt and pant legs. No children ,pets or freind/family member in the area please.
I do all kinds of welding and metal fabrication on my cars. My 76 Delta is the current big project and has a new fabricated rear frame, right rear floorboard, 2 patches in trunk, lower quarters with flares (current point of endeavers). wheel house and quater wheel lip.
My advice dont get as involved as I do, it looks easy and sounds easy but eats up the weekends and leaves you scratching your head. So I'll agree with the polarbear. Clean it up, use some bondo and fiberglass cloth, you cna use fiberglass resin with the cloth if you want but really not necessary and fiberglass resin sands harder and does not stick to steel as well. Be sure to get the cloth back into the body work so you dont sand it out when sanding. Snad the first few coats with 40 or 36. When you get close go to 80 and then 120 only before primer. I feather all my original paint out with 400 but no one else is that intence. Use at least 220 to 360 before primer, use lots of primer and let it dry for a day or two, dont use any enamel like rustoleum unless you have 2 weeks to let the primer cure, in which case rustolem is really a pretty decent product but it doesnt dry and is not a cure all. Sand primer with 400 dry which is the red paper or 400 wet which is the black wet paper before paint.
So really if you have a siphon blaster, buy a 6 in rubber sand block, some 40,80,120,320, & 400 paper. Filler and cloth, a bunch of rattle cans of primer and color, no matter what you do its better than doing nothing. It will slow the process and look better although matching the color and blending it especially with a rattle can and no buffer is another chapter.
The key to keep filler from bubbling is to get behind the repaired area which will be through the trunk in the case of rear quarters and spray some primer on the back side of the repair and then with a junky paint brush or whatever put some tar type of something to keep the condensation or water out of it. Sealing on the one side is not good enough.
Another tip I do now which would go a long way with a cloth repair also is dont try to save any of the rust damaged metal but remove it and be done with it. The rust is in the "grain" of the metal and will reappear no mater what. I now cut it right out and start at clean steel, it helps with the welding and then I dont waste all that time blasting and sanding and grinding trying to clean it up. I just remove it and start with clean steel where it cleans up. As in where the rust hasnt gotten to yet. Also what you see on the outside may be worse or spred further on the inside of the panel where you cant see.
Ill give you all the advice I can as you go its a lot of writting to cover all aspects or possibilities or everything that can go wrong to get a good result. It a real skill that thoses great body men have I know a few and Im not one of them. I can get it but it takes me a long time. Good Luck