Never use a rust converter, and if you use a acid treatment be sure you know how to neutralize it (remove the acid film)before you apply epoxy. Find out more here:
http://www.spiuserforum.com/index.php
If you don't hammer the panel you should be just fine. Keep your pressure low to start with, only use as much air pressure as what will get the job done. A good alternative to AO is crushed glass or black beauty/black diamond. Both are cheap at about $8-$10 per 50 lbs. I've blasted quite a few cars with crushed glass, never had an issue.