That rust formed between two layers of metal,where water got trapped. Ideally, you'd have to cut out a large enough rectangle out of the outer skin, to allow you to fix the rust on the inner piece,too. Then, finish repairing the outer skin.
Proper auto body repair is not the faint of heart...there's metal cutting, grinding, welding ( or riveting) involved. There are no long lasting shortcuts. Then, there's the painting aspect, too.
I'm not trying to discourage, just pointing out the reality of it.
'79 CJ5, AMC 360, t-18/d20 twin stick, dana 30/amc20(Mosers), Edelbrock Performer intake, Holley 600, ps/pb, DUI HEI, 'glass tub/1pc. tilt nose, 33x12.50 BFG AT's.
There's a fine line between "hobby" and "mental illness".