how can i fix rust?

Hello,
A few years ago I completely sanded and primed etc and then srayed my bonnet. However the rust is bad again and as the car is old I don't want to do such a major job on it this time, just do the bad spots. I know absolutely nothing about cars and wondered if you had any advice? Can I just sand the spots, put primer on and then spray the car colour on top?

Thanks and Best regards,

Sarah