If the surface is rusty with loss of primer and topcoat, the surface should be blasted again until all rust is removed. If blasting isn’t possible you can also do manual/mechanical derusting. This repair can be done locally. Briosil primer should be applied on the treated parts. After several hours of drying, the Therm-O-Signal topcoat can be applied on top of the primer.
If the primer is still in good condition, then the surface can be rubbed with sandpaper and a new layer of the Therm-O-Signal can be applied.
If the primer and the Therm-O-Signal topcoats are still in good condition you should only rub the parts with sandpaper that have changed in colour and repaint these parts with the Therm-O-Signal topcoat you wish to apply.