What's the surface profile? If it's more gritty than a 1200 or 1000 grit surface, your Blaster did you are GREAT disservice on your vehicle. I blast one time, and only one time. My media is a mix of primarily Plastic T2 granuals, a bit of Walnut, and a dash of Powerblast. When I find rust, I dwell in that area a little longer to punch it down, but, not hitting it with dangerous media that creates a profile more coarse than the 1000-1200 grit. Pure plastic, and or Soda leaves no profile in the surface, some (ignorant types) think this is 'good', I say good for what, looking at? You'll have to scuff and sand it to make primes really stick good, so, for what it's worth....
And, if you have a grittier profile than the 1000-1200 any and all rags and wax remover will shred, basically making your day miserable. Rocker Panels hide lots of media. Sand blasters have no incentive to get those areas clean as Sand is cheap. I on the other hand have every incentive to get my Plastic out of your Rockers, because Plastic is $1.85 a lb if you went in and bought it at a supply store in bulk. I blast about 1000-1800lbs over a typical B Body shell, recover almost all of it except maybe 1/4 full 5 gallon bucket.