There are lots of different techniques and opinions on this, and there is not really a right answer. My opinion is that you don't need to sandblast if you are using a rust sealer like Corroless. You just need to get off loose rust, dirt, and grease.
If you go through the trouble of having it sandblasted, you might as well have it powder coated, or use a good bare metal primer (see
this post for more on that).
Ed
