You can wirebrush to remove all loose material, powerbrushing recommended, then coat with a product called Ospho make by Skyco Cleveland,Oh.  There are other similar products out there available from autobody suppliers.  It is a phosphoric acid which converts the iron oxide to phosphoric oxide which protects the iron.  I have used the stuff for many years and have had no problems with it,It turns the rusted areas black, if for some reason there is still ironoxide, a simple coat of water will reactivate the ospho, on heavy rust, you may need more than on coat. I find spraying is easiest, but for small patches, brushing is fine, I do big trailers. It has to be primed/painted failrly soon [couple of days] or rusting will reappear.  The product does noting to unrusted steel.  Applying to hot surfaces causes the Ospho to dry to fast to work, that is when the water trick will help you out.  Hope this helps, Paul

Is this in an industrial or marine environment? Whilst the use of phosphoric acid has its uses, overcoating it with another paint can cause problems as the paint will bond with the magnatite (inert) layer and not with the underlying metal. I agree with not wanting to move equipment to blast, have you heard of slurry blasting. This is a hydroblaster with a venturi nozzle that when operated the flow of high pressure water (1500psi+ is preferable) introduces a stream of abrasive compound (grit, sand, etc) that will remove the corrosion products but also provide a decent profile (HB2+) for the paint to adhere to. In addition to corrosion product removal and profiling the water at the above mentioned pressure will remove any traces of salts/chlorides/electrolytes that will re-establish the corrosion cell(s). Generally a good deal of plastic sheeting should protect your equipment. A surface tolerant primer should be used over the prepared surface.

This has become popular in the Aussie Navy as it is more environmentally friendly than dry blasting, ie; dust, air borne contaminates, etc, also it de-salinates and provides a comparable profiled surface.