If you don't need a high-sheen finish, you can hide some of the substrate's roughness by using a topcoat with a lower sheen. Although you don't necessarily have to get down to no rust, you need to get down to sound rust before applying a primer and topcoat. That's the whole thing in a nutshell, Roy Carlsten said. The biggest mistake painters make is thinking you can get away with not removing all the rust, or at least the flaking rust. It all comes down to good surface preparation.
Most failures come form poor surface preparation.
There aren't any fancy gadgets and gizmos to help you. The Steel Structures Painting Council has issued some pictorial standards showing rusted surfaces, and what those surfaces should look like after different degrees of cleaning. Most of these are for abrasive blasting, but some deal with the more mundane methods employed by residential and commercial painters. Your inspection needs to be more than visual. Make sure you can't stick your finger through it.