If the scratch is superficial and does not affect the metal of the body panel, it is extremely easy to repair. Lightly rub the scratched area with fine sanding paste to remove loose paint particles and wax. Rinse the treated surface with clean water.
Use a small brush to paint over the scratch with exterior body paint. Continue applying the paint layer by layer until the surface of the paint inside the scratch is level with the surrounding surface of the panel paintwork. Allow the new paint to cure for at least two weeks, then sand the transition surface flush with the finish of the rest of the panel using the finest sanding paste. Then cover the treated surface with wax.
If the scratch has penetrated through the paint layer, reaching the metal of the body and causing it to corrode, another repair technology should be used. Scrape powdered rust out of the scratch with a penknife, then treat the surface with a corrosion inhibitor to prevent future corrosion. Use a rubber or plastic spatula to cover the damaged area treated with the inhibitor with putty. If necessary, and this is especially useful when filling narrow scratches, the putty can be diluted with a solvent to form a fine paste. Before the putty hardens inside the scratch, wrap your fingertip with a smooth cotton cloth. Then, after soaking your finger in the solvent, quickly run it along the puttied surface of the scratch. This will make the surface slightly concave. Now that the putty has cured, the treated scratch can be painted according to the instructions given for cases where the scratch does not affect the metal.