It's the only issue with my '87 IROC. I haven't taken the spoiler off yet but my thoughts are that the bubbling started around the mounting hole for the spoiler side piece and just went to hell over time. I could easily take the car or the picture to a body shop and ask but I figured I would see what my fellow 3rdgenner's thought.

 

Look from the inside and see how bad it is. Only way to fix it is take apart and sand/remove all rust. Repair any rust/holes and prep for paint.

opt 2. I think there are some good products out there now that will kill that rust and stop it. So the rusting will stop, but you will still see the bubbling there. You use needles and pump it into the rust. neat stuff.

Id have a good body shop nip that in the bum now before it gets any worse.

-Dan