A warning about sand blast smearing was part of my PT training class years ago. Course grit such as "Black Beauty" will dig deep into steel as much as 3 to 4 mil when SP10 or SP5 is specified. We were told to chemical clean & PT first, then blast if needed to remove surface coatings for weld repairs. If it's already blasted when you get called to do a PT note the blasted surface on your report.
Here is a link & quote.
http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/MethodsTech/preparation.htm"It is very important that the material being inspected has not been smeared across its own surface during machining or cleaning operations. It is well recognized that machining, honing, lapping, hand sanding, hand scraping, grit blasting, tumble deburring, and peening operations can cause some materials to smear. It is perhaps less recognized that some cleaning operations, such as steam cleaning, can also cause metal smearing in the softer materials."