bubbling means the paint isn't bonding to the plastic. In order to fix the problem, you'll need to re-sand and start over (sucks, I know).
In order to prevent it from happening again, I'd advise using a very diluted mix of normal
dish soap and lukewarm water to wash the case in. Do not
use dish soap with hand softeners or ones that have anti-bacterial stuff. Wash as if it were a dirty dish. Rinse it well and let it dry 24 hours before painting.
Primer is a plus, but not having it while using Krylon is probably not your problem though. Also, make sure you are following the instructions on the can, especially the temperature you are working in. 30 degree weather will ruin a paint job. I've been there, and my results were actually quite humorous.EDIT:
Said bad paint job that turned out awesome (in its own special way). This post has been edited by NiC_CC: Dec 11 2010, 12:09 AM