I want to blast some cast aluminium inlet manifolds and a couple of sumps.

What is the best blast meduim to use?

To clean? To remove material? To prep for painting?

To clean and prep for welding?

Which medium is the most abrasive?

http://www.airblast.co.uk/media.html

Olivine looks good.

ive used old blasting grit in the past with good results

I use kiln dried sand, but dont breathe in te dust! Cheap as chips and readily available

Almost any grit medium.
If you want a fine finish on the manifolds, use glass beads after the grit.

glass bead