I recently sandblasted my Hicapa frame (NOT aluminium then, potmetal)
Using recycled glass (the standard medium for sandblasting), not sure of the grit but relatively coarse; at around 90psi
The finish is very rough and needed good polishing to have some decency about it
Personally I wanted a relatively rough look to it so I didn't "polish" it, just smoothed it:
From what Read *bead*blasting would be preferred to conserve a smooth surface (or using walnut shells as said before