Yeah, even glass beads will etch away the surface very quickly. Stubborn paint or grease spots will almost immediately become high spots unless you are very careful, and use low pressure.
I had exellent results using Caswell Plating products to fill the pits after bead blasting with small particle walnut shells. I did the A/C console bezel and the 2 rear top mouldings on my 65 vert and had a 100% fill. Also i repaired a small crack of about 1/2" on the headlamp door in my 66. The trick is not to over polish the areas and to clean all pitted holes very well by drilling them just enough to remove the corrosion. Here is the link to the product. Caswell Inc. - Solder-It Products