Remove the rust. This major problem is very common for the body shop to deal with and abrasives play a significant role in restoration. Grind the area down to the base metal around the affected areas, removing the rust. Once you start to see shiny metal, stop. 5" and 6" PSA Gold discs - 40 grit; Resin Fiber Discs 36-50 grit; Body File Strips 36 or 40 grit are used to give fast aggressive cutting action.
If the rust or grinding action has created a hole in the sheet metal, often, a piece of mesh/screen or metal is used to help bridge the hole. After this has been secured to the vehicle, use body filler to rebuild the damaged area. Reshape with Body File Strips 40-80-180 grit; 5-1/2" x 9" anti-clog 80-120-180 grit; 5" or 6"PSA Gold discs 80-120-180; Sanding Sponges Medium-Fine.For the best results, sand in stepped progression to blend and level the body filler to the original suface shape. Feather edge with the medium and fine grit papers to prepare for priming.
The next step is to prime the area being repaired: Applyprimer slightly beyond the repair area and higher than thesurrounding areas. Level the surface with Waterproof 320-400 grit; Sanding Sponges - Fine. Repeat the process if necessary. Finsh sand the area with the closest grit of sandpaper that your color recommends.
The vehicle is now ready to be painted. After painting, sanding with ultra fine grits and using various compounds and polishes can remove defects and flatten the surface, producing a better looking finish. The final sanding of the vehicle should be done using 5-1/2" x 9" waterproof 1000-1500 grit for finesse sanding of surface imperfections in enamels, acrylics or clear coats. 5-1/2" x 9" waterproof sheets are available up to 2000 grit.