Remove any trim that will be in the way. Clean away all loose debris. Cut away all the rusted metal. But be sure to leave enough metal to retain the contour or body shape.
Grind away all traces of rust with a 24-grit grinding disc. Be sure to grind back 3-4 inches from the edge of the hole down to bare metal and be sure all traces of paint, primer, and rust are removed.
Block sand the area with 80 or 100 grit sandpaper to get a clear, shiny surface and feathered paint edge. Tap the edges of the hole inward with a ball peen hammer.
If you are going to use release film, cut a piece about 2-3" larger than the area you have sanded. Place the film over the repair and mark the sanded area on the film. Avoid any unnecessary wrinkling of the film.
Cut 2 pieces of fiberglass matte to match the shape of the repair. One piece should be about 1" smaller that the sanded area and the second piece should be 1" smaller than the first. Mix enough filler and hardener to saturate the fiberglass material. (Autobody Repair Jelly was used here.)
Lay the release sheet on a flat surface and spread an even layer of filler, large enough to cover the repair. Lay the smaller piece of fiberglass cloth in the center of the sheet and spread another layer of filler over the fiberglass cloth. Repeat the operation for the larger piece of cloth.
Place the repair material over the repair area, with the release film facing outward. Use a spreader and work from the center outward to smooth the material, following the body contours. Be sure to remove all air bubbles.
Wait until the repair has dried tack free and peel off the release sheet. The ideal working temperature is 60-90 degrees F. Cooler or warmer temperatures or high humidity may require additional curing time. Wait longer, if in doubt.
Sand and feather-edge the entire area. The initial sanding can be done with a sanding disc on an electric drill if care is used. Finish the sanding with a block sander. Low spots can be filled with body filler; this may require several applications.
When the filler can just be scratched with a fingernail, knock the high spots down with a body file and smooth the entire area with 80-grit. Feather the filled areas into the surrounding areas.
When the area is sanded smooth, mix some topcoat and hardener and apply it directly with a spreader. This will give a smooth finish and prevent the glass matte from showing through the paint.
Block sand the topcoat smooth with finishing sandpaper (200 grit), and 400 grit. The repair is ready for masking, priming, and painting.