Splash shields are fit to the inner wheel arches of quarter panels in order to prevent eventual damages caused by dirt, rain and salt. These vulnerable areas are impossible to be entirely sealed and metal is sooner or later exposed to destroying elements which lead to corrosion. There are cases when nothing can be repaired and quarter panels must be replaced because the rust affected a large area. Fortunately, incipient corrosion can be fixed and the panel replacement is avoided. You can repair by yourself rust spots or smaller areas of corrosion. What follows is a guide to help you get rid of rusted quarter panels.
Make all parts of the quarter panel accessible. In order to do that, strip it of moldings, trim and the inner splash shield. Wash the inside of the wheel arch so that any dirt and salt trace to be removed and afterwards dry the area. Before start working, cover your eyes with a pair of safety goggles and wear a pair of gloves.
Fit a P40-grit sanding disc to a bench grinder. Using the bench grinder eliminate all damaged paintwork and minimize as much as possible the corroded area. Do not forget to clear rust traces behind the wheel arch too. By gradual diminution, grind until you run into good metal and the rust is no longer present.
Use a can of compressed air to blow off the both sides of the quarter panel. This operation is meant to eliminate all marks of removed paint and corrosion. With a paint brush apply a full coat of rust treatment solution over the cleared spots. Leave the rust solution treatment to dry for 1 hour. This special solution absorbs into the rust and impedes later corrosion.
Utilize a piece of P80-grit abrasive paper to remove excessive rust treatment solution until you get a smooth metal surface. The inner wheel arch is hidden by the splash shield, so this action is not necessary for it.
After you placed a P80-grit sanding disc onto a sander, start feathering out the spot till the edges of the removed paint are flush with the rest of the paint.
By using the edge of a plastic spreader, cover any pits in the cleared area with a light coat of galvanized polyester filler and let it dry for half an hour. Rub down the filler with a P80-grit abrasive paper wrapped around a rubber sanding block till the quarter panel gets to its original shape. To finish the repair, add a full coat of underbody coating to the inner wheel arch.
Prim and paint the quarter panel.