First: remove the rust from the damaged part with abrasive paper. In case of tenacious rust, use a special product to remove it.
Second: apply a layer of anti-corrosive additive on the sheet metal whence the paint has been removed and plaster with a spatula.
Third: after the plaster has hardened, hone the wetted layer. Then, cover the surfaces around the damaged points with paper and spray the purposely-allocated filler.
Fourth: Finely sand the point with abrasive paper (grain 800 to 1000), previously soaked in water.
Fifth: Prepare a piece of cardboard with a hole in it of the same size as the damaged area. Keep the cardboard at a 5 cm distance from the damaged point and spray the varnish from a 30-cm distance (it would be ideal to do it when the outside temperature is around 20°C).
Sixth: For a limited touch-up, spray with circular movements. In case of wider surfaces, create various thin layers by alternating vertical and horizontal movements.
Seventh: For metalized varnishes, the point must be sprayed with double-layer transparent varnish after it has dried. You can further sand the edges after drying.