New Paint

My question is my 80 Malibu Had tiny rust spots at the bottom of both quarters. Very small I ground it down to bare metal. Put a little fibeglass over it primed and painted entire car. As I was sanding to prepare to buff I noticed tiny little bubbles so I sanded it flat and waited a couple of weeks and they are coming back. Is there a better way of fixing these spots maybe some kind of weld then grinding it down. This is the only rust on the entire car. An like I said it is very small. I did not even know it was there until I was getting car ready to paint. Paint came out very nice but I would like to fix this before I buff quarters .

Cut out the rust and weld in new metal.

yea, even if you get all the rust off that piece of metal, it will come back eventually...best bet is to cut it out and weld in a new piece