Auto body repair & detailing: rust, surface rust, primer surfacer

Question
i have 56 chev belair hd top and it has cancer on both sides of the top at the back window area about the size of a silver dollar i like to know thw best way to repair it and to keep the  rust from creeping back after i primer it.

Answer
Danny,
I'm building a '56 HT but this one's been in a shed since '71. Most of the rust was down low, in the 1/4s, rockers and above the headlights. Is the rust you're asking about an actual hole or just surface pitting? If it's a hole, the only way to do it right is trim out the rotted area until you're into solid, clean metal and mig weld a piece in there. Try to use some metal that's the same or close to the thickness of the Chevy sheetmetal and butt weld it in if possible, so there's no metal overlapping and sweating between the panels. Butt wewlding is a major pain so a lap weld would probably be okay, especially in a fairly dry area like that rear window frame. I use .023" wire and back the argon off to about 15 psi (helps to stop "burn-through"). If you can hold the patch against the backside and weld it in, you won't need as much plastic to hide the repair. If welding it on top is your only choice, try to roll your edges down so the patch will drop in a little. Weld in small tacks to keep the heat down and weld the whole seam. I often cool the weld with a wet rag or an air blower on your air hose. If it's surface rust, grind or blast out all the black pits untill it's at least 97% clean metal. Spray the area with a good self-etching primer and apply a 2 part finishing putty as needed, followed by a couple of coats of urethane primer surfacer. The Transtar people sell some good stuff for less than DuPont or PPG and NAPA sells their Econo-Body line. Good luck and let me know how it goes! Merry Christmas!