I have some rusty metal L-beams that I want to use for a project but I don't want to spend the time and money to have them sandblasted. I have heard that you can spray Hammerite paint right over the rust and that this will adhere to the metal but it sounds too good to be true. I looked at a can of this Hammer paint and the ingredients don't appear to be any different than other spray paint primers which must be applied to bare metal only. Does anyone have experience with this Hammer paint?
In general, paints like this do indeed stick to the rust just fine and encapsulate it by linking together and forming a hard surface. This is good if you want something to look good, but it doesn't do much to stop the rusting process. Eventually, the metal below the paint will rust further, and in the process expand. This will cause a crack in the paint, and eventually it will chip off. If the steel angle in question isn't structurally important, that might be fine. Depending on your climate, painting over it might buy you a few months or a few years. If failure of the steel would be a big problem, then you should sandblast or otherwise get to white metal, and pick a coating system designed for rust prevention.