This is the way I see it. I had one valve cover powder coated right, it was hot tanked and cleaned prior, it was masked off properly before sand blasting so that nothing got on the inside then coated and it came out perfect.
I had the same company do another one but it was a different guy and he didnt care. He hot tanked it and then sand blasted it with no masking and I went to pick it up and all in the inside of the baffles and on the underside it was all covered in sand. There is absolutely no way in hell you could get all that sound out with cleaning. The cover was ruined and I told them that and I didnt pay for the job and just said well, keep the valve cover because I have no use for it now.
And I specifically mentioned when I dropped it off that if it was going to be sand blasted to make sure it was completely masked off to not allow sand on the inside.
Personally the way i would do it now would just be to hand sand it and use a powerful cleaner on it and say no sand blasting.
Powdercoating is the better way to go if done right.