I have used Ospho extensively on ships. It is an acid, so definitely wear protective clothing, goggles, whatever. It will convert rust to iron phosphate, or in english it turns brown rust to black, theoretically intert and will not continue to rust. It leaves a light film which the manufacturer says that you can paint right over. In my experience using advanced paint systems (expensive paint), you shouldn't have anything there that it doesn't recommend on the instructions. So what I've done in the past is apply ospho, and then right before painting you can clean it off with a mild solvent like paint thinner. Then you get the rust removal action of the oshpo, without the residue that may or may not get along with your new paint.